1) Never. No on/off relationships.
2)n/a
3)n/a
4) I think it is most often a personality thing. I've read quite a few of these partner accounts as posts here on PC, and TBH, they very often sound textbook BPD. Makes me wonder if they're misdx'd, a missed additional dx, or just "bipolar" (in the way people willy-nilly use it, like in the way people amateurly "dx" others as NPD --narcisisstic left and right).