Russ or the Khan would not unite the Imperium. In fact, I do not believe the High Lords would recognize them at all. Likely they'd be declared some sort of Chaos abomination, the findings would be backed by an equally suspicious (and power hungry) Inquisition and the Primarchs would be hunted until death.
Yes the Space Wolves would flock to Russ and the White Scars and their successors. But neither legion was exactly very prolific in generating successors, especially the Wolves. At best, they'd have fled to the very edge of Imperial space where Russ/Khan would lead a guerilla war while trying to rapidly increase their numbers. Although for the Wolves this may be impossible since they have such a strong connection to Fenris.
Guilliman's return would be met with more respect (and fear) by the High Lords. If Guilliman called upon the Ultramarines and their successors to rally to him -- many would. But then I feel just as many would not. Why? Just because you are a Primarch doesn't mean people (or Astartes) will believe you. Guilliman's been, for all intents and purposes, dead for 10,000ish years. Again I think a great many people would just see it as some Chaos inspired trick.
We've already seen things similar to this occur in the Blood Angels Omnibus. The Chapters would not eagerly or willingly embrace their Primarchs. They'd think them some grim and masterful Chaos trick.
Edit: One thing to also keep in perspective is that amount of time these Chapters have been without their Primarchs. Yes Guilliman, Khan, and Russ led their chapters for perhaps a thousand years at most (I'm being liberal here because I don't honestly know), but these Chapters (and their successors) have been independent and left to their own devices for 9,000 plus years. In the case of the Founding Chapters I feel the Primarchs would be lauded ... but the Successor Chapters may not be so inclined since they've formed their own, very unique personalities and traditions over the years which in some cases is radically different than their Founders.
Last edited by Angel of Lies; 09-25-14 at 11:58 AM.