Hi there! It didn't come across as rude at all, please don't worry about it! A quick disclaimer before I reply, though - I do not have PTSD nor any sort of psychological training, so I do not feel I have any authority to decide if what the MC feels or thinks is a clear consequence or symptom of PTSD. All I have are theories based off how I have heard other people describe their experiences, and I would very much like to hear the opinion of a person who has PTSD regarding this topic.
Another disclaimer - while I said the explanation that the MC has PTSD or shock is valid, I wanted to make it clear that I did not. personally, interpret the MC's reactions and thoughts as them having PTSD. Like I said, I think it'd be a valid explanation but it is not the one that I would personally use.
Now since that's out of the way...
In my opinion, the MC probably has a very, very low sense of self-worth. I personally interpret their joking as a coping mechanism since it's something that I recognise in myself, but it's not a perfect explanation since, as you mentioned, the MC seemed pretty quick to forgive (which is definitely not something I would have done in that situation). They seem almost incapable of taking offense when someone yells at them undeservedly or treats them like crap - which is something I can see being a consequence of the way they were raised. A royal without any relevant opinions and used only as a pawn in political schemes.
The quickest way to diminish someone's power is to laugh about it. In my mind, joking around with Damon was a way of trying to make him think the MC hadn't been affected by his behaviour. Joking around with Bash and Aya were a way to 1. distract themselves and 2. put up a strong front. Nothing screams strength quite like a smile, I think. The trick is not letting people see it's fake. I just wish we could have seen more of the MC's thought process.
I like the interpretation that you and harmonia5 were talking about above where the MC might be trying to protect themselves in a hostile situation. Again, I still think that should have reflected on the MC's internal dialogue, but I suppose the writers meant for us to insert our own explanation for each of our MCs' actions.
Another point for me personally is that I'm a HUGE people pleaser. I can't stand the idea of being disliked and I can (through some self-projection lol) see the MC being the same, especially when they're so used to being irrelevant their entire life. It could even be a combination of both - the MC is trying to make Damon like them both because he's one of the few people they (kinda) have, but also because if he likes them he won't want to sell them out again.
I whole-heartedly agree with you on the Vexx angle. Based off what they just found out, the MC definitely should have a ton of trust issues. However, I don't have any personal experience with betrayal from a loved one (something I'm very thankful for) and have honestly no idea how long it would take for the trust issues to set in. Is it something that builds up when you're betrayed again and again, or is it immediate? Not specifically aimed at you, I'm mostly musing for myself.
Actually, the more I think about it the more I agree with you. I don't know much about PTSD symptoms besides what you've just mentioned, and it does become stranger and stranger that the MC would react so tamely to Damon even if they had PTSD. But again, is PTSD something that sets in immediately or only after some time has passed? The MC hasn't exactly had much time to process everything - it's probably not even been 24 hours since they saw Vexx.
I think the shock thing is more with the MC being numb, which is something I can get behind. Even the emotional responses are quite tame (then again, I can be quite emotional but dislike breaking down in front of people so it could be that for the MC as well), so in my mind everything hasn't really dawned on MC yet. Not even the full implications of Damon's suggestion. For now, they seem to be satisfied that Damon won't sell them off just yet, but I hope the realisation can hit them like a brick in the next episode or so. I've mentioned this before, but I really, really hope we can run away on Cursa.
So to summarize, I think the MC's reactions had more to do with numbness and low self-worth than PTSD. I think numbness is a part of shock?
I'm sorry for taking a while to reply (uni might be cancelled but their assignments are not :( ) and for maybe not answering your questions in a satisfactory way. I found myself agreeing with you the more I thought about it, haha