My question is can minimal human respect be re-gained once lost (even when not dysregulated)
The way I personally think about it, yes respect can be recovered. But it takes time. A lot of relationships here have spiraled down and down and down. Boundaries stepped over. Partner fighting unfair. Verbal abuse increases and may escalate to other types of abuse. Partners spying on reach other. And on and on. Transgression begets transgression. Often this downhill dynamic is ongoing despite the desire on both sides to have a better relationship. At one point this dynamic stops. The relationship hits bottom and may split or not. This may be a terrible bottom or the slide may get arrested much earlier. Often is starts once people seriously considering leaving, accepting that it is not worth to sacrifice everything. It starts with boundaries under the nons control.
It starts with recovering self respect.Once defensible and defended boundaries are in place the other side needs to get them. Then we need to build trust that these defended boundaries are respected even if we are not lying in wait to defend them 24/7. That takes willingness on our side to introduce vulnerability and take risk (to a limited degree) wrt. our new boundaries. Which then allows us - if respected - to rebuild trust. Which takes time.
Anyway, I was told by him that he abuses me because he can.
Right now he is telling you he can't be trusted with you being vulnerable. His words sound terrible but you already know that you need boundaries that are under your control. At least there seems to be some awareness of his behavior and self reflection on his side. It does not sound like he is proud of himself. Not that this is enough to enable him to control himself at this time.
For completeness there is the other track of us being more diligent respecting the pwBPD. When things are out of control we are usually not perfect either
Second, his go to answer for everything is I should leave & demands my plans to do so. This will happen even when he is not particularly out of sorts. It is like he desperately doesn't want me to leave, but lacks respect for me because I don't.
Fear of abandonment. He is afraid you leave him. So he pushes you away as that would be less painful than you leaving him.