Darksteel Colossus and necropotence are both replacement effects. The controller of the effects decide which applies, so yes, you can reshuffle it.
After a spell or ability resolves, both players will get priority before the current phase/step ends. So you will always get priority during the end step after the final card from Necropotence is put in your hand.
I'm actually skeptical about being able to play cards from you 11 card hand. As I always understood it there was a lose of priority that occurred when you got your +7 to hand. I could be wrong but if some one could clear this up for me I'd be thankful.