Alright.
1. The Almighty can
nullify power, it can even literally steal abilities away as well. I don't see this as PIS. Kubo legit portrays KS as this unavoidable thing. Yhwach technically wasn't under KS until the final battle. It was placed on him long ago, but the affects weren't active yet so there was nothing to null. The only PIS is him not seeing an arrow.
2. No because he can handle other people fine. Jugram's The Balance cannot be seen nor can Nianzol's ability which is simply a field of warped space around him. It's implied Uryu's ability can in theory tussle with Yhwach but that's because Antithesis can pretty much reverse the events between 2 targets.