Damn you have been hanging out with Nurikabe I'm sure of it
. That is a huge compliment by the way. I pretty much agree with all of your post and I don't think I can add too much. I just want to address one thing and then answer your end questions.
I do think it's a little off putting how hard she pushes for the lifestyle on some of the low-C/low-D routes, but it's not like she stops touching her husband (usually a feature of this sub-slut wife trope). I agree and its this sort of thing that makes it very hard to trust Steph and believe what she has to say. The dev said that as well, I would probably not believe Steph's big secret since it is Steph saying it. I'm prepared to accept it because the dev has said it is the truth and he created the story. We could put Steph's reaction down to NRE (New Relationship Energy for those who don't know) and accept it on that basis, but the switch flips far too quickly even if you do nothing with Donnie until the Saturday, the switch has flipped. Maybe its my distrust of Steph and the devs red herrings and I could be reading too much in to it, but it doesn't sit right with me. I feel there is more too it.
Now to the thought experiment. This is fun BTW. "You find out she lied about being a nymphomaniac but felt terrible about it, she comes clean, and says she just wants you in her life no matter what your future sex life holds." Younger me would have been naïve enough to accept that at face value. Older me would say, that's what every woman who deceives her husband says (especially cheaters), and they nearly always go on to do it again. She may even believe those words, but I just couldn't trust her and no trust rules out any relationship for me.
I don't want to be checking phones and locations and shit, too old for that. Honesty, trust, loyalty, respect and honest communication are vital. I could also be spoilt as I have those in my marriage now. Living an alternative lifestyle doesn't worry me, we've lived one for years, but I won't be disrespected and humiliated and my wife is even more fanatical about it. We are on the same page as her first marriage was very similar to mine. Once you open that door for someone like Steph, you are deluding yourself if you think you can close it and go back to a traditional marriage. That is just asking for heartbreak. So to answer your first question Yes, no and no.
The nymphomania is a massive red flag. To accept that would be to accept she will cheat and do so multiple times. The fact she is willing to deceive me to get what she wants and I let it go, means she will lose respect for me and I would lose respect for myself. The chances would be extremely high she would do it again because it would be easier to do it again the next time he thinks she has a 'good reason' and the next time after that and so on. The big thing with both of those scenarios is there is no trust. I'm not going in to that sort of relationship again. Younger me would have been stupid enough to believe I could change her. Older me is too old for that shit. So to answer your question, both.
I may seem harsh but after the shit I have been through and am still going through (22yrs after the divorce), self-preservation kicks in.