I just had a really, really, really weird succession

I'm playing a zoraostrian ruler, I had the kingdom of Khiva, it was under elective.
My preferred heir was my eldest son, I have another son

I created the kingdom of Persia, it started as gavelkind
My king died before I could switch the inheritance laws and then everything got freaky

The Kingdom of Khiva?
Destroyed
Every. Single. Vassal I had, boom independent, even the ones in Persia

My eldest son?
No titles for him

My youngest son?
Yup, he's now the king of Persia

Anyone know why this happened and how I can avoid it?

Even stranger It says I have no heir of my dynasty, yet my half-brother, who is of my dynasty, is my heir

EDIT:

Also any ideas how I might fix this?
Khiva and Turkmenia were all dukes, and the center of my powerbase, so now my troops are roughly 1/3rd of what I had before
The dukes won't accept vassalization, even though they A) love me and B) I have a claim on their de jure title and C) I'm 10x their size
All the counts accepted vassalization, but I can't reach the counts I had in Alan, due to the distant realm penalty There are two big Muslim kingdoms right on my doorstep that might just invade me