don't know bout the double shield for fire and kurenai. Danzo barrier is 3 turns unless overwrite. We haven't gotten the new group summons yet and shield do stack but iruka only shields konoha ninja while mabui only shields water and light ninja though I do not know if there is a variable for the shield health difference.

Manda and the others haven been added into the game yet so you're gonna have to wait til they patch it in in the later version of the game. and yeah auto barrier ninja's are speed based the faster you are the more likely you will have to barrier buff