Oh boi. I feel sudden urge to share my emotions.
Long story short: After ~6 years of playing this mod, countless evenings and night, cycles of forgetting-deleting-reinstalling, and innumerous events IRL finally did it. I spawned American rebels as England in new world, and switched for them mid-game, thus recreating famous events lol.
I spawned as England (Queen Elizabeth ftw), gigantic map, with ~20 civ, Snail speed. Also BarbCiv ON, with New World Policy set to 5 (BarbCiv in New World settling only after Old World civ build a city on a landmass). I also tweaked BarbCiv to became Minor Civ with slightly less chanse than default (bigger city), and to keep being a minor slightly longer. Nonetheless, there are around ~35 civ's in the world now, with ~5 or so dead.
Well, it wasn't particularly easy game, there was some challenge, but still I managed to create a large state and became first in score, with huge economy and few "core cities" with enourmous production (typical stuff), and few vassal states. I created religion (Christianity, because King's Richard Crusade is OP) and spread it far and wide, up until 25% total world influence when I finished Guttenberg's Bible (but then it dropped to ~20% for now, as religion decay is always on for me, and there are two other major religions that follow my footsteps). Still, AI is usually leading in tech, at least in certain branches, especially that one state (Arabia, lol) - at one point I sent in spies like mad, preventing them from developing Liberalism before me. I'm not sure why I did this, probably because of free tech, or maybe just mad lol. I failed eventually because of distance, they are literally on the other half of Old World, fastest ships took around ~25 turns to reach there, plus movement on shore and mission preparations. But I managed to hault their progress for like ~40 turns.
But, eventually, I nearly caught up with them, and I felt as game became less challenging. I played a few up until late Modern/Transhuman era before, I knew where it will go - I usually start steamrolling everything with little-to-no effort from late Renaissance (usually with countless Crusaders and Royal Knights upgraded with state-of-the-art flintlock rifles). Untill there is no opposition, and I lead up in every way possible, have a train of vassals and economy beyond reach of mortals.
BUT!
Its turn 1940/3600, year 1760 (so, slightly earlier than IRL). Revolution is in the air, I pressed the button to play as rebels, and I have to do it all over again. On a barren landmass outside from the rest of the world, with like, 7 small and empty-ish cities, surrounded by colonies of former myself and other powerful civs. And probably I have to build Statue of Liberty, can't be MURICA without it. Granted, now I have Noble difficulty (instead of Immortal, on which the system kept me for, like, forever). Hope FlexDiff will not bug after civ switch or something :/
Either way, thank you people for developing/supporting this mod for so long. Even after Civ 6 this mod could provide something truly unique.
Few notes/questions.
1) For some reason I'm not in a war with metropoly? Well, I can press ALT+lmb by myself, but still...
2) It seems that min/max FlexDifficulty settings do nothing. Both for AI and for myself. I've set it between Monarch and Deity, but early in the game there was a moment when FlexDiff changed it as Prince for me. And AI just don't give a damn - there are plenty of loosers with 1-2 cities, with Settler difficulties.