Well... I rant into that situation before, I autosaved/loaded a few times with varying outcomes.

1. Lost the game.
2. Since my Comnenus were still around(though ancient farts), they tried to take the throne.
3. My king lived the two years it took to get Prince John to King John. Oddly, the King lived to a ripe old age of 87 and had a few kids.
4. My favorite one of all was the King John event. My king died and his 14 year old son was crowned king. This is the rarest varient of the no valid heir event.

The fleet goes to your side since the rebs can't have ships.