Razor One this is an interesting scenario but the OP need fleshing out, you say we understand how their weapons work but can't replicate them due to a lack of the necessary elements but how does this translate to the rest of their technology?
Shields and Thrusters are electrically driven, so I'd say it's a matter of power generation. Fusion reactors would be necessary to adequately power most of their technology sans of course weapons which as stated require an exotic substance that cannot be reproduced on Earth without some major teching up and infrastructure.
Can we recreate their non-FTL drive techology?
Yes. If not the Benefactors are willing to manufacture drives on our behalf until we figure it out (more detail below).
Can we recreate their armour and repair the mothership? The way you've written that sentence implies that although weapons are out we can manage most of the rest of their materials, excluding wormhole drives.
That was my intention. At the very least we can provide some of the elements to them that would require extensive mining.
Wormholes are very tricky though. We could probably understand the theory given time for our scientific community to absorb it and a convenient nearby wormhole to study. The materials and equipment necessary to safely travel through the wormhole however require significant technical knowledge to build and maintain. This works in our favour as the Benefactor vessel will have the necessary brains trust to repair or rebuild such a device in the event it gets damaged.
That said, it is possible to travel through a wormhole without the necessary equipment. Just don't count on having your internal organs remaining internal by the time you get to the end of the ride.
Who is onboard the vessels of the Benefactor fleet, is it just the crews and they're using the technical databases on their ships to try to give us their technology or did they scoop up their surviving best and brightest before hightailing out to the backwaters of the western spiral arm of the galaxy?
Inquiring minds need more to work with.
It slipped my mind to include this information. Here's what my notes say.
Mothership Crew: ~38,000 (55,000 full complement, casualties sustained in battle).
Support Staff on Mothership: ~24,000 (Maintenance, repair crews, service staff, etc)
Soldiers, Pilots: ~11,500
Officers: ~1800
Science Personnel: ~700
With regards to their fighters, they have 8 squadrons comprising 24 wings themselves comprised of 24 fighters for a total of 4608 one-man fighters capable of being dispatched at once with four squadrons worth of pilots (2304) on standby for rest and relief purposes.
The Support Staff figure contains its own organisational structures, department heads, foremen, overseers etc. that keep the maintenance and manufacturing capabilities of the Mothership in shape.
The science personnel are included on any typical Mothership. Motherships were built and designed to be largely autonomous, capable of sustaining long range patrol and deep strikes into enemy territory, as well as providing firepower and support for shorter ranged vessels such as Cruisers and Benefactor Dreadnought-Equivalents.
To this end, the Mothership contains manufacturing and production facilities necessary to reproduce most of their technology and spare parts. The primary problem facing this particular Benefactor Mothership is ammunition, which requires advanced planetary infrastructure and a decent respite. Prior to their entry into the Wormhole they were chased mercilessly by the Enemy, the Wormhole being their one safe point. For now.
Getting back on track, a science staff is usually included on any Mothership class vessel for obvious reasons and is generally well rounded enough to handle most anything they come across.
1) What is the extent of the Enemy's holdings?
Their "Home Systems" comprise a small fraction, approximately six percent, of the galaxy. This is what I would think are areas that are well settled by them, secured against attack and of course devoid of any rival sentient species due to extermination.
A further six percent is within what could be called a "Zone of Exclusion", a region of space where they've effectively glassed worlds containing sentient species where they maintain outposts and fledgling colonies.
A
further six percent comprises the territories once controlled by the Benefactors and their allies. Currently, half this area has been glassed, invaded or garrisoned, the remaining half is still under control of the Benefactors allies whom are doing their best to give the enemy as hard a time as possible.
2) What is the Enemy's population?
I'll be honest here and say that I couldn't begin to fathom their population. With the amount of area they control I'd venture a guess of trillions at least... and I'm probably off by an order of magnitude or three given the volumes I've conjectured above.
3) Does the Enemy have Von Neumann machines?
They prefer to do their killing themselves, so I'd say no, though the technology wouldn't be completely outside their consideration if they felt the need for it. Their prime attribute, beyond abject xenocidal mania, is arrogance. If they had any degree of consideration towards Neumann machines, they'd think them crude and unworthy at best.
4A) How many ships does the Enemy fleet have, how fast can they replace them, and how big are they?
Something I couldn't begin to guess at myself... enough to cover their areas, burn out their exclusion zone and facilitate an invasion of Benefactor territory.
In relative terms, the Enemy has technological parity with the Benefactors (Regular non-wormhole FTL drives included). The enemy won their war by building up a massive military numerical advantage. The Benefactors estimate that at the onset of the war the Enemy outnumbered their forces ten to one.
In terms of size, Benefactor Motherships range 10 Km long by 4 Km wide, with fighter bays, shielding, armour and big guns that have a limited amount of ammunition. Cruisers are of similar proportion and configuration, 1 Km long by 400 Meters wide. Cruisers can re-arm and re-fuel fighters but have no hangars or bays where they can be maintained properly.
Enemy Dreadnoughts average twice as large with corresponding firepower and crew. Most Enemy vessels range smaller, with Dreadnoughts tipping the top of the scale for size. Their thick armour and heavy armament make them slow moving juggernauts.
Benefactors utilised finesse and precision strikes to bring them down. The usual tactic is that, once a chink in Enemy armour appears, hit it with everything you have until you either irradiate the crew or cause a catastrophic chain reaction.
As to how quickly the Enemy can replace their losses... they're currently defending a massive area, patrolling another massive area and are still fighting over yet another massive area. Even with their massive numbers I'd say that they're stretching themselves a bit thin. If, for example, an equally powerful enemy were to launch an attack now, they'd be forced to pull back most of their forces to adequately defend their homeworlds. Their economy is definitely on a war footing and has been for some time though with their recent conquests that may have to change once they've secured their new territories.
4B) Do those ships require rare materials or exceptionally difficult construction? If so, how would that scale to something that was easy to build, like say a communications sattelite, in terms of tons per year?
Bit more detail than what I'm privy to... I'd have to say that their hull armour is almost certainly a difficult to produce alloy for our current metallurgical knowledge... if they indeed use metals at all (Composite armour perhaps?).
I'd say the more difficult challenge would be reproducing things such as engines and shields. If you can manufacture shields then you generally don't need to make your ships ultra-tough unless you expect for your shields to go down. Until our materials science catches up and as long as we can reproduce their shield technology (or have the benefactors use their fabrication equipment to manufacture them for us until we can) I think we can afford to have any vessels we build, whatever their purpose, made out of current generation materials.
5) What is the biggest telescope the Enemy can build?
Fairly large, though I imagine such telescopes would be space based. I'd have to picture interferometers the size of solar systems at least... if they can exist on such a scale of course... possibly even larger.
The way I see it the Enemy didn't get to be top dog by being dumb and uninterested in the universe. They're cunning, curious and hate all intelligent alien life. I would have used the word "Xenophobic" but that implies a level of fear that does not exist.
A little further background information.
The Benefactors have been developing wormhole technology for some time but only recently did it become viable in the sense that they figured out how to get their test pilots back as more than vaguely organic goo.
The Benefactors managed to effectively bluff the Enemy for about a decade or so with the threat of "Wormhole Weapons". Think of it as akin to the US being able to threaten the Soviet Union with nukes before they obtained them. In the meantime, they tried to their best to unite as many species in their sector behind a mutual defence pact. While many did sign on, mutual distrust and prejudice prevented the cohesion necessary to sufficiently rebuff the Enemy until they had managed to establish significant forward positions. While the Alliance did eventually stabilise it came too late to save the Benefactors main worlds.
The Benefactors planned to use wormholes in the event of a war to deliver fleets past enemy blockades and sensor nets, swallow particularly troublesome worlds and drop them into interstellar space or dump them into the interior of stars... effectively a transport system and a weapon of mass destruction all rolled up into one.
Unfortunately, much like Germany's V2 programme, it became a viable weapon too late in the war to change the outcome. It did however lengthen the war by several years as the Enemy was forced to redeploy some of its forces towards self defence while the Benefactor fleets, what remained of them, managed to carve some deep wounds into the Enemy.
This has effectively bought some time for their Allies. Though they don't possess wormhole drives themselves those that remain are fighting for their survival and are willing to throw out old hatreds and prejudices... at least for now.
A few clarifications.
Benefactor technology has parity with Enemy technology. The superior numbers of the enemy and lack of cohesion in their alliance is what did them in.
The enemy is currently spread thin and trying to consolidate their territory. If the 1 - 3 year assault fails, the travel time in the century scenario is assumed to be a best case retaliatory scenario barring wormhole travel (assuming it's been either closed or effectively locked down with conventional forces) with the ship / fleet being dispatched immediately and assuming they encounter no empires between us and them willing to bar their way. I should have made that clarification in the OP and didn't think that through. I'll be honest and say that I didn't do the homework there.
In the case of a siege, the Enemy is willing to take steps to eliminate us while preserving what they can of the ecosystem. They tend to value life bearing worlds if not the intelligent species that inhabit them.
I'd like to apologise for some of the more vague points in my OP, in all honesty I should have drafted it a few more times before posting. That being said, some points came up during the discussion which I had not honestly considered so I guess those flaws would still have been there regardless.
One possibility to consider in the vein of running... what about retreat into the Earth? If Benefactor armour is capable of withstanding the forces involved then conditions in the mantle shouldn't be too difficult for them. I'll admit that getting down there is going to be tough as hell but once there you've got a significant meat shield in terms of the Earth's crust, plenty of geothermal energy and nigh unlimited metal resources. If they try to bake the Earth's crust off to get to you, they lose the biosphere they value, if they come down to get you they're going to have a significant bottleneck... and that's assuming they know you're there in the first place. I don't see their sensors as capable of penetrating hundreds of miles of rock and magma and I don't see anyone remaining on the surface (assuming only a vanishingly small fraction can make it down of course) being willing to give up your location after we've managed to cover our tracks. Going underground, quite literally, may be a viable option if it's absolutely impossible to win in the near term.
I apologise for the length of this post but I wanted to get things out in a bit of detail. I've made a few educated guesses and a few not-so educated guesses. If something I've mentioned is outright impossible or completely off, by all means, call me out on it.
