1) colonization takes place after civilization has reached the point where individuals can have "lifespans comparable to that of stars." (

and following from that
2) the colonizers are motivated to try and stay in touch with the mother civilization.
To the first, while it is infinitely more plausible than FTL, it is not a given; I don't know about you, but its seems to me to be a whole lot easier to just construct a seeder ship(s) (especially if we throw in Von Neumann self replication

No.
In fact, it would be rather detrimental to your argument because both of the two approaches for drastically extending lifespan (extending the life of the body with gene tech or robot bodies or flat out uploading peoples minds to a computer) eliminate one or another of the historical factors for colonization; specifically, population growth and population density * .
Now, in a civilization where your physical body can live for tens of thousands of years or is otherwise functionally immortal, then its a safe bet this civilization has already mastered the art of population control. The only conceivable reason people in such a civilization would have for leaving their home system is if they want to get away from their mother civilization for whatever reason (civil unrest, war, discontent of some kind-- possibly with the population controls themselves

Think about how different the U.S. and the U.K. are. Now imagine how different they would be if communication never improved past snail mail.
Furthermore, if you go for the mind in a chip approach, you have now effectively eliminated population density from the equation. The efficiency with which this civilization can use the space it has to house people will go up by so many magnitudes as to render colonization a pointless exercise except as a way of creating a back up copy for disaster insurance. If they even go through with it, then there is no need to keep in contact with friends back home-- you can just bring along copies of them and you will never know the difference. Freaky, but that's how that kind of civilization works.
And of course, if you use the classic setting with humans colonizing worlds and living lifespans no longer than modern Earth, cultural unity is a lost cause. But you knew that.
* well, historically the third factor would be greed... but considering how ungodly expensive it is to move materials interstellar distances, I think we can agree that this isn't going to be much of an issue anyway. Not without inventing some really, really contrived bit of handwavium.