jmart is codependence by circumstance to me, and I 100% agree fanon Martin is a huge mischaracterisation. That being said, I’m also with you on exploring the canon-accurate relationship there — what does it mean to be clinging on to each other as a last-ditch effort to keep a semblance of humanity?
Jon is with Martin because Martin keeps him grounded. Martin only likes the “human” part of Jon - the part he can easily wrap his mind around - and so belittling the “monster” part of the man is no problem to him. Without Martin, Jon would’ve succumbed to avatarhood wholly. With Martin “keeping him in check”, he’s only toeing the line and can be (harshly) pulled back a bit, if necessary.
Martin stays with Jon because it offers him protection. Without Jon, he would be likely to succumb to his own little Lonely domain (which we get a brief view of). It’s also stated that by staying with Jon, Martin is shielded from the physical aspect of the horrors - he’s still scared by them, but they cannot hurt him or wound him, and so he can walk freely through the Fearscape in the role of a viewer. It’s the best case scenario for him.
Also, by staying with Jon, who is by default more monstrous than him, Martin gets to reassure himself - by looking at Jon being very far gone, he feels more stable in his own slowly deteriorating humanity (he’s ashamed and afraid of the fact that he, too, has a domain, which makes him the perpetrator following the Fear-infested world logic - he cannot bear the thought of being just as harmful as the other avatars, who he deems terrible monsters that should be killed)
They don’t have many other reasons to stay together, because they simply hadn’t had the time to get to know eachother as people outside of their grotesque eyepocalypse hiking trip in which Jon is actively being consumed by an eldritch being.
In the half an episode we got of them pre-apocalypse, their relationship seems to have had only started blossoming. There is nothing more than the usual kind exchanges between two friendly people. It’s a nice beginning for something bigger, but it certainly isn’t love, not at that time.
After the apocalypse starts, there is suddenly a lot of weight put on their bond - they are quite literally the only ones left. They cannot afford to be casually lovey-dovey when their wellbeing and sanity are at stake.
Had each of them been on their own, I’m sure that witnessing the sufffering of countless people would have its effect on their mental health; it’s also very likely that they would’ve turned into avatars of their own. By staying together, they are also distracting eachother from the horror surrounding them.
Their days are spent walking through fear-filled landscapes, not to mention that each of them has to deal with their own problems - Jon has suddenly found himself in a position that makes him both very powerful and very fragile, and Martin is fighting off his desire to isolate himself and his fear of Avatar-Jon. Every time they say “I love you”, all I can hear is “please don’t leave me”.