So it seems that struggles can be set up to activate after the game starts, but the struggle has to be pre defined. You aren't going to be able to have dynamically generated struggles, the only way a struggle will be able to activate is if a region already pre-set to have a struggle fulfills whatever parameters it might require. At least this does theoretically mean that something like a Crusade struggle could happen if a mod pre scripted such a scenario to occur, but only in those limits and with what the script says the struggle is.
I'm still curious as to whether there can be multiple struggles at the same time(or rather, whether multiple struggles can even exist at all in the same world state), that question has yet to be answered it seems.