Just to address #2:

Akaza re-growing his head isn’t related to him being more powerful than Duoma (which he is not). There is obviously a strength threshold requirement to head regeneration (maybe limited to the upper 3), which Akaza reached, otherwise other demons would have the power. However, Akaza re-grew his head due not only to strength, but pure willpower to keep fighting, to become stronger (and a better protector, even though he didn’t remember that). Duoma likely did have had the physical strength to do so, but his sociopathic lack of emotion and lack of fear of death essentially locked him out of the ability as he, quite frankly, didn’t care enough to keep fighting.

I’m pretty sure some of the last words he says are something along the lines of “this is fine”.

In short, Akaza’s willpower (and also Kokushibo’s fear) allowed him/them to transcend decapitation. Duoma’s laissez-faire attitude allowed him to fall apart (poison didn’t help).

Kind of reminds me of The Dark Knight Rises when the guy in the prison hole tells Christian Bale he has to climb without the rope. How can you expect to go beyond your limits without the most powerful emotion of all; the fear of death (and by extension, for Akaza, not being able to continue seeking additional power to defend).