This isn't Neto- anything. It's a one-sided crush and a hidden relationship... Swear to god you all can't use terms for shit.
Anyway, "sitting down and having a reasonable discussion" proves to be the unusable ultimate skill, once again. Seriously, they're all adults... I think. Not sure about the short mage girl. Either way, Lin has all but secured the party's destruction by being an idiot. The idea that Lin even considers him dating Sophia as him "stealing" her from anyone is misogynistic and basically invalidates her agency in picking a partner, and I don't know if the story will ever actually address how gross it is for him to think so little of her. The best I can see happening is Lin 'gains enough self confidence' to stop thinking that he 'isn't good enough for Sophia' instead of recognizing that Sophia was more than capable of making her own choices in deciding who she likes. Cain 'liking Sophia first' doesn't mean jack shit, never meant jack shit, and will continue to mean jack shit.
You know what actually DOES suck for Cain? Harboring feelings for months towards a person who's keeping their relationship a secret from them. Not to say it would ever justify any negative actions, but it certainly would be more of a reason for him to be pissed off than Sophia 'daring to' date someone other than him. Cain thinking he's a "better man than Lin, thus meaning more worthy of Sophia" would be just as bad as Lin thinking he isn't, since he would genuinely just be objectifying her into some kind of trophy to be won through 'hard work' instead of mutual feelings. If Sophia ever liked Cain in that way, I think she would've been entirely capable of turning down Lin.
Another thing to note is that Lin appears to hold absolutely ZERO faith in the foundations of their relationship... He and Sophia have been dating for literal months, I believe, and yet he somehow thinks that Sophia would be so incapable of staying faithful that she would actually drop everything they had just because a "better man" showed up with feelings for her. Could you imagine being Sophia and having your partner hold so little faith in your fidelity? That would be such a fucking insult.