Yes, that's true, it becomes his power, but the power of "black" doesn't harm or affect his opponent unless:
1. the ink released from Ichimonji comes into contact with its intended target,
or:
2. Ichibei performs Futen Daisatsuryo.
I wonder if Ichibei can complete the ritual before Yama slashes him with ZnT?
Ichibei was not obliterated. Obliterated would imply there's nothing left. He was simply dismembered an reassembled. Just pointing that out.
I think it could go either way, honestly. I still am under the belief Yama's destructive capability is unmatched, just yet. Ichibei might win in terms of technical prowess, though.
Yes, Yama used his entire bankai against Yhwach, and it had different properties at the time. I kind of wanna see it in a flashback if possible.