I know Le Stag already said he's rewriting this last chapter, and I think that's a good call. Nathan has some nice good-guy dom vibes. Like cozy ntr for mild subs. The dirty talk when it's going on, but then the "hey man, I'm doing that because you and your girl are into it, we're cool, right?" after. Having Oli as the 'hard' option and Nathan as the gentler option works really well. That could even play well in a threesome, maybe with the MC having the prompt to decide if Nathan's influence wins out and tells Oli to "calm down, dude" or having Oli win out and Nathan goes harder "Oh okay, if that's what you're in to..."
Having Nathan as a co-cuck is a huge shift and feels weird, but I think the concept could actually work by introducing a new character. A weak, wimpy coworker who's always lusted after Laura at work? Someone the MC can let Laura tease and deny, either to satisfy Laura dom tendencies for a sub MC, or an MC dom fetish.
Still hoping the Ashley path leads to the MC sending some humiliating photos to her. Him after cumming on himself. She can send pictures of her with a hung hookup, and demand he shows how he measures up. Laura could fill that role too, but it seems like more of a shift in that relationship.