I don't think the amount of timeline fuckery he could pull off here should be taken as representative of his casual abilities.
He'll try to kill us sure, but I doubt he can do stuff like teleporting back in time to abort Viserys. If he could pull shit like that then he would have done so now, instead of fighting us while we were capable of harming him.
As for handling this situation right now, we don't actually have to let anyone remember this. Wild Arcana'd
Damnation of Memory would let us steal their recollection of whatever we do to fix this and finish the oath. We only need to get enough people to make the ones we miss sound crazy to reduce the wave to historical footnote instead of major miracle.
I think we should go out of our way to to do this, since the Chronomancer would have known we could block this. The point probably isn't (just) petty spite, I bet he's putting something together to exploit our reaction to his benefit.
Putting things back on our original plan's tracks (or as close as we can manage to them) is probably the best bet since that's the timeline he's trying to avoid.