The plan was indeed to starve the chaos gods, but he didn't consider how disbelief actually worked. Energy was still transferred into the warp, just instead of being in forms that could feed the current gods ("mystery", "rage" and "hedonism"), it was in a form that could feed a new god ("disbelief").

The Emperor was always an obscenely strong psionic force, I assume you mean the pre-throne astronavigation? He's a strong beacon despite being dead thanks to the throne somehow keeping him together, without the whole death part he wouldn't have needed such a crutch to project his power. Part of his spiel was also that he's the incarnation of everything man could be, so the ability to "draw on his power" would have already been in people. As I understand it that's part of the distinction between an aspartes and a custodes.