Don't go in unless you've beat all 3 games: Spoiler
I had a brainstorm waking up this morning -- I connected the dots between Shep being basically dead as a doornail in ME2 and then the Shepard VI stuff that's in there as a throwaway.
FACT: There exists a Shepard VI being sold illegally. It's "corrupt" as in it's obsessed with reapers.
FACT: We know Shep was pretty damn dead from various cutscenes in 2 and 3.
Cebereus Scientist in ME3 cutscene:
"Shepard is clinically brain dead. After that much trauma, that long with no oxygen...we cannot overcome nature."
and
"The helmet kept the brain intact...for whatever good that will do."
Shep herself mentions while watching the scenes above:
"I don't remember anything. Maybe they really just fixed me, or maybe I'm just a high tech VI that thinks it's Commander Shepard."
FACT: Catalyst at the end of ME3 states that Shep herself is partially synthetic, so she'd be destroyed by the red "destroy" ending.
FACT: Miranda Lawson wanted to put a control chip into Shep's head. Why would she want to do that? Unless....
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What I think happened is that Cebereus created a very advanced Shepard AI as part of Project Lazarus using detailed psych profiles, video footage of Shepard, and scans of the original brain tissue to fill in holes and then used the AI to "reboot" Shep from a brain dead state once they got her body going again, with maybe brain implants to replace the totally destroyed bits of brain or to act as a "bootloader" for the brain.
The AI somehow got leaked into the wild and since it's trying to run on low end public hardware; it defaults to Shep's obsession -- STOP REAPERS with one liners instead of being capable of full sentinence, so people assume it's a poorly programmed VI, instead of a sentinent AI trying to run on shit low end sega genesis hardware.
It also explains many of the "do I know you?" conversation options for Shep in Mass Effect 2/3 -- the Shep AI doesn't have 100% of the knowledge that TrueShep does from ME1.
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944
That's, honestly not a bad theory. I really wish ME3 had EDI show that footage to Shep sooner in the story, because I also thought it was a fascinating tidbit.
I still think Player Shep is the real one though, because the Illusive Man was ADAMANT to Miranda that Shep BE Shep, and not a clone or a simulation. He needed Shep to hold up under extreme scrutiny, because he needed Shep to be a focal point for resisting the Reapers. I always took the "partially synthetic" bits as stuff that Project Lazarus simply couldn't repair, and figured the bits wouldn't cause doubt about Shep's identity (I THINK the intro scene shows part of his/her spine is artificial at least).
And if Shep was an AI or advanced clone or something, Miranda and/or EDI would have spilled the beans on that.
THAT SAID
I think the clone from the Citadel DLC fits the bill for this. Clone Shep to get a source of emergency organs, and flash-imprint their AI on it to make it not a drooling mess, the AI is still buggy and obsessive and breaks free after the real Shep wakes up.
"A person is smart. People are dumb, panicky, dangerous animals and you know it. Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know...tomorrow."
-Agent Kay
Shep in ME2 and ME3 also "jacked into" the AI world on two occasions; something somewhat strange since a standard human shouldn't be able to "jack into" machine worlds.
A.) The final boss fight in OVERLORD DLC in ME2 to fight David in a Virtualized world.
B.) During the Geth/Quarian campaign in ME3 when you jack into the Geth Server.
As to how they "loaded" the Shepard AI into Shep's dead body without any telltale "giveaways" such as implant jacks? The renegade glowing red eyes in ME2 are a clue.
Perhaps they used the optic nerve as a bootloader /root access to the brain stem via ocular implants. The ocular implants could also be something that could also be explained away during a physical -- you suffered explosive decompression and the freezing cold of space, so your eyes, one of the most susceptible soft tissues of the body, would have taken a big hit; so it would not be out of the unusual to have a sort of regenerative matrix placed over them, or a synthetic cornea over them.
That's what causes the glowy red eyes. It's also how you "jacked into" virtual worlds twice during the series.
Basically, I think:
Commander Shepard: Version 5.1 of S.A.M. and Release Candidate 1.0.1
Shepard Clone in ME3: Version 3.2 of S.A.M.
Bootleg VI on Citadel: Version 1.7 of S.A.M.
It explains as you say, how the ShepClone was able to go from decanting to plotting to take over your position in the space of just six months, instead of being a drooling retard -- yet at the same time it doesn't quite have everything that's you yet -- witness how it doesn't understand what it calls the "Cult of Shepard".
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944
It's outright stated in ME2 that Shepard is a cyborg with implants beyond what was necessary to repair the damage. It's why you can pick up the Widow, an anti-armor rifle meant to be fired from a bipod or other vehicle/fortification mounted position, and use it from the shoulder without injury or really much issue controlling it.
They tease the issue of Shepard being either in part or entirely a reconstruction via VI/AI, but I'm fairly certain Shepard is not, based on the ease the Reapers have had with subsuming more advanced AIs than those developed by the Citadel races at the time. The Geth were taken over fairly easily once the Reapers arrived, save Legion, who was not only unique but had spent significant time with Shepard as well.
That said, resurrecting a dead brain doesn't mean that it is the same consciousness, and I really doubt continuity of consciousness occurred given the circumstances. I think a more realistic theory would be that Cerberus used an AI to put the original pieces back together in simulation while they repaired Shepard's brain to make sure it'd work, and that's what leaked out.
I think there are two things Catalyst is trying to do when it suddenly says "hi there" and offers you options.
Motive/Goal #1: He's trying to outsmart you and indoctrinate you similar to how the Illusive Man and Saren were indoctrinated.
It says: "Sure, you could destroy us, BUT it would (insert list of dire consequences), and also destroy synthetic life in galaxy...you are partially synthetic as well." to start to sow doubt in your mind about the only rational option.
It then offers you two alternatives, which are disguised indoctrination -- Transcendence (the same trick it pulled on Saren in ME1), and Ultimate Control (same trick it pulled on Illusive Man in ME3), and carefully couches its words to make it seem like those two are not-bad alternatives to the catastrophic picture it paints of the DESTROY ending.
Motive/Goal #2: He's trying to run out the clock. You know he's doing this, because when you reject all options and get the ultimate bad end; the Crucible's beam goes out and Catalyst says "Welp, this cycle is done."
Basically, when you docked the Crucible to the Citadel, Catalyst went "what the?" because no Organic has ever made it to the Citadel and started the sequencing of the main gun; so Catalyst doesn't know how to turn it off.
But...it's a hyperintelligent AI, a million times beyond what we could perceive. So it knows that if it can stall you for a couple minutes, it can crack the encryption on the command codes for the Crucible and shut it down.
Basically, he's lying to you to save his own ass, and keep his programming going.
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944
Despite the rocky nature of the ME3 ending, there was still so much potential in what they did present to us for something truly great to have emerged. I do really like the Indoctrination Theory that is out there. It's cohesive enough on its own, but with the rushed chop-job the ending turned into, a lot of plot leaps and gaffes could easily be attributed to the theory.
Or Shep's (heh) theory that Shepard is actually an AI. If they had plunked that nugget into the plot sooner and dove into it, it could have been one of those really, really fascinating conversations, especially if the story left it ambiguous.
The Catalyst was always tough for me to reconcile, because it suddenly forced the Organics/Synthetics conflict to the forefront (where it never really was). Keep the Reapers an unknown, Lovecraftian-style threat.
"A person is smart. People are dumb, panicky, dangerous animals and you know it. Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know...tomorrow."
-Agent Kay
I’ve read some fics that posit the reason that the Shadow Broker was working with the Collectors (and Reapers) was that as a Yahg, being restricted to their homeworld, they would survive this cycles’ reaping, and if the Shadow Broker took care to deposit a technical archive on the Yahg homeworld, similar to how the Prothean Beacon on Thessia was done, the Yahg would be next cycle’s Asari, except a lot meaner.
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944
The ME2 opening of Wilson suddenly deciding to blow up Lazarus Station and you awakening to gunfire makes a bit more sense if you think about the fan theory that...
...the Citadel is performing low level indoctrination on those on it.
Basically, it’s not enough indoctrination to make people into mush; but more of a low level “uphold status quo” indoctrination -- witness how the council refuses to see what's going on until it's too late, and even during the war, the station's population is a bit too "calm" for what's going on.
As long as the Reapers stay in the shadows, and use their cutouts -- the Collectors – they can avoid people from taking an active interest in them, and keep the low level "status quo" indoctrination going as long as possible.
A Sovereign class reaper blowing through a mass relay on it's way to destroy the Lazarus Station (and Shepard) would attract a lot of attention, especially if the official government line was that Sovereign was a one of kind Geth dreadnought.
Given that the Shadow Broker was getting actual video from within Lazarus Station; he had the project penetrated pretty well; and the Collectors/Reapers probably paid him for the interesting technical data related to Shepard's body/rebirth.
But when it looked like Shepard might actually come back -- the Reapers paid the Shadow Broker to put a stop to the project.
Stage 1 of Stopping Lazarus was Wilson deliberately waking up Shepard early, that almost destroyed the project, but Miranda spotted it fast enough to save it. This was basically the “keep cover” attempt, if it had worked, Wilson could have plausible deniability.
Stage 2 of Stopping Lazarus – was the “fuck it, we don’t care if your source is burned”, where the Reapers paid Shadow Broker enough money to entice him to switch to severely active measures to stop Shepard’s resurrection.
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944
And a lot of the issues with the Reapers and Catalyst are solved when you realize that they're basically a "Paperclip optimizer" AI; but not an "on the fly" self editing unshackled general AI like EDI is.
Basically, the Reapers/Catalyst can slowly evolve over hundreds of cycles, but they don't do well with sudden abrupt changes in events; and they're rigidly fixed by their initial programming parameters -- "prevent conflict between organic and synthetics".
"If scientists and inventors who develop disease cures and useful technologies don't get lifetime royalties, I'd like to know what fucking rationale you have for some guy getting lifetime royalties for writing an episode of Full House." - Mike Wong
"The present air situation in the Pacific is entirely the result of fighting a fifth rate air power." - U.S. Navy Memo - 24 July 1944