For a given value of easygoing, indeed. I don't think Sass would stop the ambition, but might focus it in interesting ways.

Definitely not denying that Hawk Moth is capable of all of those thing, as there's more than enough demonstration of cruelty and temperament in what we've seen. Mayuri suggests that the compassion potentially extends past the immediate family. Or, perhaps, that either Mayuri is seen as family. Or it could be that Hawk Moth needs Mayuri to help make sure plans go well and are thought out enough that they pass the five year-old test on be Evil Overlord List.

Paon Bleu as a night flyer makes as much sense as any other headcanon, given how little we know about the Miraculous and how it came to its current state.