To be believable, a story doesn't have to be realistic in every single respect. To suggest such a thing is preposterous on the face of it. It would mean that any fantasy or science fiction story which involved anything improbable would automatically be a bad story. Superhero films? Automatically bad stories, because they're not realistic in every respect. The Lord of the Rings is a bad story, by that standard. But who is honestly going to suggest that the single most influential fantasy story in history is a bad story?
To be believable, a story simply has to have a premise which we can understand, characterizations which we can understand (and preferably with which we can identify), and internal consistency. We can find a story about "the chosen one in space" (Star Wars) or "the magical chosen one" (Harry Potter) believable, because they're well written and conform to certain basic rules. It doesn't matter in the slightest that things happen in those stories which couldn't happen in the real world.
The suggestion that all of our real world problems must always follow us into our fantasy escapism completely invalidates the foremost purpose of fantasy escapism. I therefore do not find your argument to be compelling.
So, your first point went completely off the mark...But you sort of brought it back to with the second point, so the fact that you know both of these things and yet chose to make the first point... Bro, really? Yes, a good story, regardless of the setting, or the genre, sets down it's own rules and follows them through, leaving behind little to no plot-holes. Realistic within it's own setting does not mean it has to conform to our own reality. In that regard, most, if not all, superhero movies and the better part of fantasy and sci-fi stories, fail to deliver. They often break their own rules for the sake of the plot. I mean friggin Star-wars? Really? It's an entertaining saga, but calling it a good story is quite the stretch.
As far as JOHN is concerned, we've already established that Connor is very busy and that Jess's relationship with her next of kin is, at the very least, loving. Considering this, I don't see a scenario where Connor cheats with Jessica's mother or sister, or where either of the latter agree to stab Jessica in the back, regardless of her own cheating.
I actually think the open relationship idea is a good one, but way further down the line in the story. Before that happens, there are a bunch of hot, sneaky cheating scenes/affairs behind Connor's back that have to play out first with Mr. Parker, the fat ass mayor, and others. Then maybe in the future Connor discovers Jessica's adultery after she meets the masked vigilante and starts an affair with him "behind Connor's back", not realizing that it's Connor she is cheating with. Depending on how many points you accumulated with Connor, you would have different choices - 1) Connor keeps it to himself and gets revenge on Jessica by cheating behind her back with mom, sis, etc.; 2) Connor confronts Jessica and they break up; 3) Connor confronts Jessica and they stay together and agree to an open swinging lifestyle.
This though, I'll admit I find rather plausible and may be the only reason those two NPC's were introduced to the story, aside from the lesbian incest that Stopper unfortunately denied having interest in from the get-go.