The best suggestion is to go with a smoke test. The worst part of that for me is the strange smell it leaves from cooking the vegetable oil to make the smoke. It lingers for about a week.
Logically I would say that it is impossible for the gaskets to now be leaking but frankly I don't recall ever seeing a chart with a number specifying the maximum pressure that intake gaskets are expected to hold?
Quoting from Shelock Holmes on the scientific method of "deductive reasoning", "when you eliminate the obvious, whatever is left must in fact be the answer".
To me, that simply means you have exceeded the holding pressure that the intake gaskets are designed for? Sorry to state the obvious.
I did a while back have a set of intake gaskets I got from my engine builder that were hard, not soft like the FelPro's. They seemed to be an "asbestos" type of material? I had them hanging on the wall in the shop for a couple of years and when I went to move them, they broke due to lack of flexibility. I never saw another set like that.
I would also have to add to the list of considerations of possible causes, "resurface the heads", but I would need to see with my own eyes if even the stock "turkey tray gasket" could cause this? Your description indicates multiple leaks which to me makes no sense whatsoever?
You win the prize of "I stumped the 'Peanut Gallery' "! Congratulations!