Oh god please help.

So I'm King of Wales right now and I've started to expand into Ireland. I'm one county away from being able to found the Kingdom when 2 of them randomly join some other realm. I figured out that my vassal died and his heir is a vassal of this other kingdom.

I went back, hoping that he'd live, but when I check on him it turns out he's got cancer. I'm not allowed to revoke the title, and I can't change a crown law allowing me to for another 9 years. This guys heir is also seemingly well liked, as the highest i can get my plot power is 29%.

I've spent 20 years conquering Ireland back from these fucking petty kingdoms, how do I prevent my work from going down the drain?