Revolts should happen whenever they have a good chance of success. New ruler is not as strong as old one, and if vassals revolt en-masse - they will have better chance to overwhelm the liege. Or would you rather have late(when the ruler is strong and full of power) revolts with only one OPM-count revolting? WOuld that really be fun for you?
Perhaps I didn't make myself clear enough. Revolts should, obviously, happen when they have a good chance of success, but with two reservations: not "whenever", as in absolutely whenever there is a good chance of success, and they should sometimes happen when the chance is poor, e.g. due to despair or miscalculation or bad intelligence.
As for one OPM revolts, I believe that just shouldn't be happening without very good reasons, such as: reasonable hope to set your empire crumbling down, your tyranny being unbearable, OPM having good allies, OPM ruler being stupid.
Right now positive opinion means a lot. if your vassals love(do not despise) you - they will accept the offer to become your vassals again right after the ultimatum - they have proven their power, but they still want your protection.
Not the distant ones. You will get the distant realm penalty, as well as "not de iure liege". That's already practically too hard to beat, even before you add "small difference in rank". For example, if you're Poland, there's no way the Duke of Aswan or Gallecia will bend knee even if he's your total fan.
Speaking of which, people who are at 90-100 opinion, especially with Honest, Just, Kind etc. traits shouldn't be revolting against their lieges. If you're a law and order person and you're totally committed to your boss, you just don't rebel simply because the opportunity is good. That's nation-state thinking, which isn't really applicable to appointees from your own culture or even dynasty in far-off lands. It should probably take several generations before they become full of self-determination ideas.
That, and when they're surrounded by Muslim powers who will DoW them them the moment they DoW me, they really shouldn't rebel. If you're some European-born Duke of Aswan between the Caliph in the North and a united Abyssinia held by your European king's co-dynast in the south, then you really, really don't want to rebel if you're smart. The same goes when you're a duke of Gallecia or Portucale with emirs all around. You want to be on the best terms possible with your king and you want that king to be strong enough or else you'll end up paying the jizya tax at best.