A couple of thoughts after reading through all your great reactions:
1. I loved the ending. I never think that endings are long enough personally. Not even Lord of the Rings ;-) I would always gladly read more. I really liked the book, but it the other two in the series were better.
2. I kind of thought that Invocation might have been a different being of the same race that didn't suck at his job like Calamity. He started the powers and then left like he was supposed to. He didn't get an F.
3. As far as how the powers work here is my theory:
Calamity has the ability to create custom viruses in his body. These viruses are delivery mechanisms for his genetic changes, and they are keyed to specific DNA sets. This would explain how children could inherit, but also twins would get the powers by infecting each other. Maybe he delivers the viruses by touching people, and that could add or remove a particular power set from the target. Also this might explain how some people like David and Mizzy fight the powers, but when they are particularly under stress they snap. Their body gives in to the infection.
How is that for geeky.