This was thought-provoking and well written. The way you described Severus’ affair with the shopkeeper was very intriguing and kind of hypnotic. I particularly liked his reaction to almost getting caught. For a moment there, I was sure Harry had seen the whole thing. Would he have been able to hide it so well, though? I’m still undecided.
I think Severus’ infidelity was a symptom of some unresolved issue(s) in their relationship. (Why else would he keep going back for more?) Lack of proper communication, perhaps? Boredom? Loneliness? Harry certainly seemed to have become unable to talk about anything besides his work. Why? I also got the feeling that Severus was emotionally cold and incapable of expressing his love to Harry. They both suffered because of that.
Maybe I‘m reading too much into it, but Severus doesn’t convince me here: The apothecary's shop provided fine goods, along with such common pleasures and conveniences that had been absent from most of Severus' life. He'd accepted them with relative prudence and moderation, and he thought it must be wisdom, this new indifference to the incidentals he'd been offered and had enjoyed. I feel sorry for them both. Hopefully they’ll find a way to start discussing their problems before it’s too late.