i think a large part of where fan creators go wrong with wallmark - more specifically wallter - is that they all make the same mistake of assuming that wallter reminiscing his past relationship with mark = wallter is still in love with mark
the reality is that no matter how they actually feel or felt about the relationship, no matter how much they hate their ex partner now, people in real life will reminisce a LOT about past relationships- especially a marriage that lasted for six years. and on TOP of that, wallter and mark have probably known each other for a very long time even before that. its not unrealistic, nor out of character to assume that wallter sometimes misses what he had with mark and remembers an idealized version of those golden days of their relationship, while also absolutely hating mark and not wanting to see his face ever again.
truly, the person who loved mark is gone now. the moment he let himself fall into the influence of gray stuff, which may have stuck him into a point of no return. wallter and mark can NEVER happen again, not by apologizing, or making up, or talking about each other's feelings- it just is not possible unless you completely ignore wallter's gray stuff addiction and pretend it's not killing him. he literally just doesn't care about anyone or anything that isnt a building made of concrete.
its also very important to remember thst just because he may miss his relationship with mark sometimes, that doesnt mean he feels that way ALL the time. its also a very common realistic experience to suddenly be hit with guilt and regret when thinking about happier times in the past that you can never get back- only to later on realize why they ended in the first place, and it turns out you dont actually miss those times all that much.
feelings arent linear and they arent permanent and they are absolutely NOT supposed to be simple to understand. it lacks realism when you think of feelings and emotions purely in a conventional way. theyre adults, it only makes sense that they have complicated adult feelings and cry about stupid things sometimes.