I don't really know, but a wild guess would be: Is the "earlier bookmark" mod aware of the changes done to provinces by the other mod? Because otherwise, it would try to load a history for a map that is 77 provinces bigger than it expects, which could cause all sorts of weirdness.
No idea about the cultures thing.
Thanks for the reply
I'd made my provinces quite-high numbered to avoid any issues/conflicts if new Provinces are added, but apparently a max_provinces greater than 1650 results in a slow-down on loading, and anything more than 1700 seems to result in the infinite loading bug. I had to re-number my provinces, but now they work with about 1-2 minute delay on first load.