So, here's a question if you're okay answering it (I'm mostly asking because I love If Living Can Be Like This)... Outside of the moral issues, does Song Lan ever stay up thinking about how Xue Yang kinda... basically killed his entire family? Does that ever get talked about (either with Xingchen or Xue Yang?) [Preemptively sorry if it has in the fic and my sieve of a brain forgot it.]
this is one of those things that I feel like I should deal with and just...haven't. (there are a few of those. I know they're there. I just have been leaving them alone, because I didn't feel like it.) anyway! your question!
the answer is yes, and no, respectively.
Starting with the latter - it's something I think he wants to talk about, sometimes, but doesn't feel capable of doing so because of the can of worms he knows it would open. Because with Xiao Xingchen, it's so tied up in Xiao Xingchen's guilt and the pain of their break and how much Xiao Xingchen blames himself for what happened, and bringing that back is...a painful thing to imagine doing, both for himself and for Xiao Xingchen, and what would he say?
It'd just make Xiao Xingchen guilty and miserable, and it wouldn't, probably, make Song Lan feel any better. He doesn't know what he'd want Xiao Xingchen to say or do in that situation that would do any good, or make it hurt less, or make him feel better about what he's done/is doing.
And with Xue Yang...Song Lan is realistic enough to know that Xue Yang doesn't feel remorse for things, at least not in the way that he conceptualizes it, and he certainly wouldn't express it that way. He just doesn't. So what would be the point of that conversation? Xue Yang almost certainly wouldn't apologize. If Song Lan asked him if he regrets it he'd probably say no. If he asks if he'd do it again he'd probably wiggle out of answering because how am I supposed to know, do you mean if I went back now or if things went just the same then or what, and just generally would be uncomfortable and trying to get out of the conversation as quickly as possible, and sometimes when Xue Yang is uncomfortable and wants out of a conversation he starts biting.
So again: what is he going to get out of that conversation? What healing or resolution would come from it? It'd just be strife and upset with no relief.
And a-Qing...Song Lan loves her but he's not exactly going to talk about his dead temple trauma with her. That's just not a conversation he's planning on having with her, either, for several reasons.
So there just...isn't really someone to talk to about it, which is a pretty sad thing, and something that's not easy to deal with. I don't think it's constant, though - and this is related to the first part of the question you're asking. I think it comes in waves.
It's not like he's continually thinking about it, at least not after early stages when he starts to, oh no, get used to this situation. It gets easy sometimes, if he lets himself, to not think about it - but then of course that's a betrayal of his temple. And he keeps betraying them, too - and when it was just for Xiao Xingchen's sake that was one thing - when it becomes about Xue Yang then...that's something that's much harder to deal with, or to feel like he can justify.
I expect he lies awake at night at least sometimes thinking about this, and not feeling very good about it.
Ultimately it comes down to that same decision he made initially: that Xiao Xingchen's happiness was worth making a compromise. He made that choice, even though it meant setting aside revenge/justice for the dead of Baixue Temple. And now...it's certainly a complication in his Xue Yang feelings. But unfortunately his Xue Yang feelings have gotten complicated, even though he was actively trying not to let them, and there's only so much fighting that he can do.
the guilt's there, though. It's just something I think he tries to deal with on his own. not that he's all that good at hiding it, and I think Xiao Xingchen would be able to at least guess part of what was going on but wouldn't push him on talking about it (partly because he doesn't want to).
