Should I attack the hands in rashomon?
I can end the battle by targeting ibaraki first. Does the hands contribute to the boss health bar?
No, you should prioritise on dealing damage to Ibaraki first. You can try to clear them using AoE NPs, otherwise you bully the oni first.
If you're confident on killing Ibaraki, take care of the hands. I usually do this after Ibaraki has about 10k health left and can be killed in 1 turn.