Borderline is not the same as manic-depressive/bipolar disorder. It's easy to get then confused since the names are kinda similar, but bipolar disorder, which is what Kanye has been diagnosed with, is characterized by periods of extreme energy (aka mania, sometimes the person doesn't even sleep) and periods of deep depression. Whereas borderline is characterized by having really strong emotions towards everything, which usually makes the person have really black and white thinking, ie. that things/people/events are either amazing or absolutely terrible with nothing inbetween, also often accompanied by a low self-confidence and fear of abandonment.
Kanye has been diagnosed with the former but judging by the way he's been acting recently he perhaps *could* also have the latter, but I'm not a psychologist so I can't really say. In any case the man needs professional help and not to be milked by media outlets, be it left leaning or right leaning ones.

Yeah. There are different interpretations to their relationship but I like to imagine them as Wednesday and Enid in the Wednesday series. With a bit more Stockholm syndrome I guess. Again, depending on the interpretation.

Eh I'd say nobody likes feeling either unloved or inadequate. But yes doing something like this to embarrass someone is a red flag.