I think there should be a limit though when it comes to the willpower thing with Ningheim, allowing them to use multiple ways to channel willpower is a good idea but there needs to be something that limits them in some way, that I have yet to really decide on.

As for the angels, a decade would be a good position to go from. I myself would say 20-35 years max since they arrived, resistance has risen and fell multiple times, each providing minimal change if any.