Well, in order,
In this setting souls are a natural force like gravity
Nobody ensured life death and rebirth before the gods came along, and the gods don't ensure them now
The gods are extremely powerful, and presumably able to incarnate.
Wael is basically the god of ignorance, yeah. Or really, the god of always asking questions
The Engwithans were already gone, all Ondra planned to destroy was their ruins. Thaos probably would consider that a necessary sacrifice to keep the big secret, but whether he approved or not he's not the boss of the gods
One thing I like about the portrayal of Ondra as extremely stupid and unpleasant is that it adds to the final reveal of the gods being creations. People frequently say "what does it matter where they came from, they're still gods" and the answer is, well, they're ****
I don't think the natural laws of the setting would allow someone to absorb all that energy and gain power. If they do, then I'd expect them to save "become a god" for the big ending of a trilogy or something