Antonia, I so deeply relate to the post you wrote. So much of it is broadly similar to my own experience that it is hard to pick out specific things that ring true. But wearing the masks is something that the therapist picked up on straight away.
My xh handled the whole divorce thing very differently - I think I have commented before on at least a couple of patterns they exhibit - one is to be 'nice' to varying degrees [if you discount the little matter of abandonment and infidelity!] In this case want desperately to think well of themselves. Your xh seems an extreme form of this. My xh displayed the opposite, of being really hateful to me and the children. [There are lots of spewers!] During the divorce process he lied, delayed, refused to answer questions he was obliged to do so until my lawyer had to get tough, and generally did all he could to give me as little as he possibly could. It was really spiteful. I refused to allow it to hurt me or see it as personal.
My therapist said he had seldom encountered anyone as deeply guarded as my xh, as out of touch with his real self. He is, and remains in deep denial, and it causes him huge conflict.
My xh also said he had been pretending to be happy, that he always felt like an outsider looking in and not able to join in. There was a pre-MLC family joke that xh was happiest in another room, knowing that his family was nearby and he could join in at any time. So yes, always a little detached.
I think many of us know that oftentimes if the MCer could put the clock back and not have done this,they would not now do it, but having done it, as you say, the path of least resistance is the one they take.