It was the O-rings after all.  

They were deteriorated to the point of not holding the valve seat in place. I feel so stupid, I should have checked them properly in the first. I just didn't know how tightly those valve seats should be seated. Well, you live and learn. Main thing is that the bike runs like new now.  

Yep. Makes sense that after a fire in the carburetor everything rubber is done for.

Good news!
glad you're back up and running...