I think I can agree that Mankar Cammoran's speech is not true, most likely, it's just the rambling of a delusional madman. After all, anyone who would knowingly help a divine being whose sphere is destruction invade his own world can't be sane. That's sort of like burning down your own house while you're inside it, accept instead of fire you have a god and instead of a house you have the world.
As to the second question, I'm not too sure, all I can say is that it probably would be a very unpleasant experience to those living on Nirn, a lethal one as well, most likely.
Actually, it's more like you burning down your own house while you're in it with the promise of immortality. To some that's more important than happiness I suppose.
And to woever mentioned it, all Men & Mer are relatives, if not children, of the Aedra. Without the (in the eyes of Mer) trickery of Lorkhan, the essence that makes up all of Nirn and it's inhabitants would still be one with those who are the direct descendants of Anu and Padomay. So technially, Everything in TES holds something in common with everything else. With out the sacrafice of Y'ffre(Earthbones), there would be no life on Nirn. Or without the trickery of Doom Drum, the Aedra would've never created Men or Mer of themselves. I see Lorkhan as an Aedra. Not a daedra.