You can build the 'crusader-church', can't you? (And also the crusader-state-proclamation should be in place)
If you have this building in the settlement, then characters in the settlement should go 100% christian again (after one turn), at least it is triggered like that ... while you anyway should try to build every christian belief-producing-building in such a culture-room.
Also, it is triggered, that only non-royal-family members can get a foreign belief trait (at least as far as i remember now what i coded at last, means, the heir, the leader, and born sons of them shouldn't get any foreign belief trait).
Anyway, thanks for your report. Please playtest this campaign further up to 1140/50, if possible.