From what I've understood, that was a drinking/hotel party he was obligated to got o since he was tasked with taking care of the director. One can argue he shouldn't have been there in the first place, but Korean (and Japanese) work culture means he didn't have much choice in the matter. He didn't do anything sexual with the nurse (technically) and gave her a cursory breast exam. Again, if I've missed something, please let me know, but as I understand it, there was no actual cheating until after the divorce.
Same thought. Althought both parties know that the marriage is failing, the ML didn't do anything that would be considered as cheating while in the middle of marriage. He encountered both females while he was still married but he didn't do anything sexual nor intimate with them. I mean that's what I think hehehehe ┑( ̄Д  ̄)┍
They really didn't communicate at all; I think they're both to blame for the failure of their marriage. I do think the husband may have cheated emotionally before the divorce, but as far as physical cheating goes, there's been no evidence of that yet. But who knows! Future chapters may say/ imply otherwise.

I've only read What's been uploaded here so, someone correct me if I'm wrong, but the husband isn't technically cheating yes? He started fucking the cellist and the nurse after they divorced?