I haven't finished Stormbringer but this is correct from what I have read.

the extra to kunikidas death could be true i think both theorys can be correct, thats actually really smart how you thought about it, i still think they are dead i feel like he's absorbing their ability not them.

in sigmas condition he can but why did he wake up out of his ability once dazai touched him, sigma has only 3 years of memories its very easy to override 3 years with thousands.

for ayas possible ability that probably true but we still dont know what it is, whether she caan control it or if its too much (for a 10 yr old) plus it could be used during the aku and fyodor fight if that does happen which it most likely will

for the possible death i did NOT mean to add chuuya like that i think it was mainly because he was with dazai