hi sluggo and

this kind of behavior (having sex during separation/divorce) is not at all uncommon or limited to relationships that include a borderline partner. a close friend of mine is going through a divorce and has done the same a few times.
i understand this is all very confusing. a person with BPD experiences two conflicting fears, abandonment and engulfment. a diagnostic criteria of BPD is frantic attempts to avoid abandonment. in essence, that is what you are experiencing. she gave you a very profound, unique bit of insight when she said that she would start having negative feelings of you when the lawyers start to litigate. this is common as well. do you share children? you may want to have a look at our Family Law board, and if you share children, our co-parenting board.
as far as your specific questions go, i recommend having a look here:
PERSPECTIVES: Relationship recycling [romantic partners]