So let's compare Abby and Kelly. As you said, Abby tortured him for one day to make him snap with the intention of him realizing that Lacey is his problem and he can take that to a therapist to help him heal. Note that I do not think her intention out weighs what she did.
Kelly is consistently hitting him with small mental attacks with the intention of making him snap so she will sleep with him.
Kelly is being selfish by trying to harm the MC for her own gain. Based on the intention she says, Abby does not. I am not completely sure that party of Abby's goal was to get him to leave Lacey to make him available for her sister, but that is never said. Still not a selfish goal.
Both are insanity.
I try to avoid conversations here about cheating. Mostly because it is a very subjective subject. What one person sees as cheating is very different from another.
Honestly, I am setting here trying to think of anything Lacey did in Act 3, that I would consider cheating and I can't off the top of my head. I don't and will not consider the stuff she did while she was roofied as cheating. I will agree to disagree for anyone that thinks otherwise.
So, if we narrow things down to sex (intercourse, BJs, and HJs included). Heck we can even add kissing into this. Since they were married, how many men has Lacey slept with outside her marriage? Of those how many of them happened while she was sober? Next how many of those happened without the MC knowing about them before they happened? Please don't think I am saying any of those were justified, I'm just saying lets list them.
Now ask those same questions about the MC. The questions are harder for the MC because alot of his are options.
I have not dug through the game to validate these numbers and I don't know what they are off the top of my head, but these numbers are close if the MC is not more.
If we talk about flirting, I'm not sure the MC is capable of talking with one of the girls without flirting with her.
I want to stress again, that I am not saying Lacey has not hurt or lied to the MC. However, I have seen growth from her in every act and she still has a long ways to go.
The problem about the what constitute as cheating would depend on a mixture of intent and actions, first she wasn't sober when she cheated on the MC with Damian, however she clearly went to that bar with the intent of picking a guy up to cheat on him with to prove it 'didn't matter'. That is absolutely cheating, she took that decision very much while sober, days in advance. Everything with Jared could be construed as cheating, she didn't have sex with him but she let him get away with several intimate physical acts. When she kissed baldy at the club, before Damian saying how kissing was special to her, its clearly cheating, I could understand kissing even on the mouth family members, there are peculiar traditions like that, but a stranger? Nope that is cheating. In Act 3 we learn that after telling to the MC she'd go back to him, she fucked Isaac, which again as many argued would be considered cheating, because at this point she had consented to be in relationship with the MC, and she betrayed that. At the Dance club she literally asked her husband if she could dance with the guys, saw he was frustrated about it, and went on with it, the fact she was clearly lucid enough to ask him about it and question him clearly about it, mean she knew what she was doing, so she effectively cheated on him because their wasn't given consent for her to do it and yet she did it. She hadn't taken more Ex afterward, yet at the strip club, and the BDSM club she also did whatever she wanted even if it was clearly without her husband consent and at his visible frustration, so cheating again.
So what did the MC do so bad? After she cheated on him during his free pass week he initiated possibly a few things, and after the free pass week was over he kept in some cases it going, at this point can he really be blamed? His wife had cheated on him and the more time went on the more betrayals from her he discovered, so he kept being involved with 'his girls' if they wanted it. He was entitled to so much when you consider everything Lacey did to him, that I don't see a problem with everything he did, plus it was Lacey's stated intent to sorta settle him up with a harem to some degree, so he just kept that going, while he never agreed to her doing anything with any other men, which she did.
Next issue, their is so much left unsaid because we don't see in Lacey's mind all that much, Lacey clearly as some interesting notions of what matters or not, she said fucking another guy doesn't matter, so does it mean she doesn't consider it cheating? So if that is the case how much could she has potentially held back? Even if she does agree that vaginal or anal sex would be cheating, does it mean she think the same of handjobs or blowjobs? Did she think like Clinton that oral sex doesn't count as sex? If that is the case did she give a ton of blowjobs that she considered as not counting for sex to not being cheating? Their is so much left unknown and we can't trust a single thing she says, so its possible their is alot she did that she left unstated, from the moment she figured 'I shouldn't say something if its going to cause grief to my husband' God knows what she is holding back from him. Was Isaac really the last guy she fucked before the MC or did for whatever amount of time in between that call and their reunion she considered again it didn't matter and just kept having sex to deal with when she got horny?
One of the main issues here is likely if you are a fan of NTR, NTS, Swinging & Cheating, you might have a very limited definition of what cheating mean, because since you don't mind the notion of your partner fucking around you are quite free with the liberties you think are fine for your partner to have, if however you aren't into all those things, your definition is likely very wide & strict, so those divergent point of views could very much cause massive differences in what is considered cheating or not.