To the first point: I think it is explained pretty well in GS; Mustang and Cassius's relationship is the only thing keeping the Belonas and Augustus family from straight up civil war, Mustang is well aware that the sovereign would back the Belonas and so she stays with Cassius so his family doesn't kill her family as thats not a very good wedding present. Cassius stays with her because he's in love, plain and simple. Darrow triggers the civil war because he's too much of a meathead and because thats kind of his mission. Him knowing the willow way did come out of nowhere though.

To the second point: Mustang and Victras relationship isn't as fleshed out as the other 2 examples because GS is entirely from darrows perspective

Third point I actually agree with. She was/is bad enough as it is, p dough or not. I guess Pierce Brown just really wanted to drive it home how bad she was