I think in terms of Jonmartin and jealously:
People don't actually crush on Jon a lot, so a lot of time Martin knows he's being kind of silly and Jon is playful about it because he's not going anywhere.
But then every once and while Jon meets someone he just connects with, because that's just the type of person he is. You either love him or hate him, and when someone loves being around him, they really love being around him.
And that makes Martin's jealously from "kind of silly," to something more serious that Jon is probably going to have to be really reassuring about, and even that likely isn't going to solve the whole problem, because Martin's jealously isn't really about someone liking Jon in general, it's more about the fear of abandonment. He worries that Jon will like someone more than him, or be happier in someone else's presence, or even leave him for someone else. And Jon does make genuine attempts to reassure him, but the doubt is something Martin really struggles with, so sometimes he has bad days where he can get caught up in his feelings about Jon seemingly connecting with someone else in an area which he "failed" at. So jealously is a struggle for Martin.
Then I think Jon is on the total opposite end of the spectrum, where if someone hit on Martin he would be more likely to just raise his eyebrow a bit, maybe have some light banter, then move on with his day. I don't think he would really read into Martin's innocent interactions with other people, or even have big reactions to Martin being flat out flirted with. Which I think could cause an issue in itself, where Martin see's Jon as "not caring," because he doesn't get jealous in the same way, but Jon's had more relationship experience and therefore he's just more settle into the dynamic of a relationship in general.
Plus, unlike Martin, I think Jon's self worth issues are more the loathing sort. Where Martin worries that he's not enough, I think Jon's self worth issues are more so along the lines of letting other peoples opinions of him having a large power over how he see's himself, guilt for things he limited control over, and unease about his wanning humanity. I don't think of any of THOSE issues would turn into jealousy. I think he's more likely to have "What do you see in a monster like me?" moment then a "Did you enjoy talking to that man more than you enjoy talking to me?" moment, where I think Martin is vice versa because his trauma presents very differently from Jon's despite being similar on paper.
And yeah I don't really know how to end this post. TLDR: Martin would probably struggle with jealously even if Jonmartin could be together in a better world. Jon probably wouldn't, because his trauma affects him in a different way.