My instinct is telling me that he cannot stand the fact that he doesn't know how I feel since his engagement came out.
It sounds like there is shame and guilt and he has struggled with it. While he got involved with another women, there was no down time between relationships to grieve the ending of the relationship with you and it appears that was painful for him.
Is this normal behaviour for them? Do they try to get your attention in weird ways like this? Or is he just that clueless about how he's perceived by my friends and family - but surely, his comment on 'not wanting to be judged' shows that he knows too well that he's been judged for his entirely horrible behaviour towards me?
No one has (or can) predict BPD behavior to this level... .but we can read the actions at face value.
It sounds like the they (exbf and his fiancé) share some stress over the way the relationship started. Starting as an affair is not the Cinderella story most women aspire to. This has all likely permeated their relationship at a few levels - insecurity on her part, shame on his, some illegitimacy of the relationship to both of them in the eyes of others... .
It could be that they are now stepping out from under the black cloud.
1. Deleting photos of you on his social media is most likely for the fiancé - a closing the door to the past that he was holding onto in some way.
2. Her contacting you to apologize is likely a result of the two of them talking about his (or both of there) shame and wanting to get closer on it. Its sounds like he or he/she want to be forgiven.
3. Opening the door with a mutual friend - is part of coming out - relieving shame.
4. Applying for the job is most likely also about coming out - relieving shame.
My instinct is telling me that he cannot stand the fact that he doesn't know how I feel ... .
No one ever feels good about be "no contacted".
You had strong healthy values and boundaries and conviction to them. That will serve you well.
Things changed after he told his mum we wanted to get married, but that's a different story.
What happened here?