No, Packie's just a cynic. He left LC abruptly and seems to have cut off all ties from there, so he really wouldn't know.
Lester said Niko "went quiet" because at the end of the game (in both endings) it's implied that Niko is done working for the mob and other gangsters and has no interest in returning to that lifestyle.
Yep, pretty much spot on with this post.
I like to think he died though. I like to think the whole depressing tale of gta iv ended in just as depressing, misserable circumstances - maybe he cut his leg on a rusty nail, it got infected, turned to gangrene, he lost his leg, fell further into depression, got drunk on vodka, tried to light a gas cooker, fell asleep and gassed himself to death.