He only mentioned cooking dinner to me once, but I thought he wanted me to go and hunt some game. I'm not sure if there's any game to hunt in Rivington, but I had a look... and I guess he doesn't like the food at the Elfsong. Or the fact that I'm still serving up 3 week old camping rations every night

I don't think Mizora even commented on it at all for me. I suppose just complaining that I'd made her look like an idiot would be something, given that Wyll had nothing to do with the rescue and it only worked because Mizora wanted to showboat and rub it in Wyll's face.
I figure the Blade of Avernus stuff is Wyll's attempt to get revenge on Mizora and all devils. At the time he made that pledge he thought she'd killed his father to punish him, so it made sense in my game. Did he talk about becoming Grand Duke before then? I can't remember how it came up, but I remember telling him "You're not a politician, you're a hero," so that might also play into it.