False! If two Hive-fleets meet they will eat each other. The stronger will eat the weaker and become stronger.
I'm sorry to tell you this, (I'm not, really) but the Perpetual lore is no longer valid. The whole thing about the Emperor returning has been abandoned by Games Workshop. He is mortal, he can die. And he will. The lore about the Emperor being the Star Child is like the 5th Chaos God, Malal. It was never officially made non-canon, but everyone considers it invalid and irrelevant at this point.
Are you fugged? Ollanius plus, Vulcan, and the emperor are all confirmed to be perpetual a on the new horus heresy novels.
Yea he is immortal. its not dropped. In fact the perpetual lore is at its strongest right now. Most of the recent hh books have had perpetuals. And the Emperor is one of them.
You really seem confident of it. Chaos all but won during the heresy, yet in the end, the Imperium prevailed. DO not assume things will happen, unless you know for sure its gonna happen.