Getting back to Hunter a second, ultimately his presence is required on both a Doylist and Watsonian angle.
Watson wise, he is the one drugging the women, thus allowing for the MC to fuck them, and is the one who pushes the MC to fuck them.
Go watch What If 7, the whole thing is based around the MC never knowing Hunter's plans, and he's a passive character there, doesn't work out since he doesn't need to spend time vigilant with Catherine, doesn't open the OnlyFans with Brenna since he doesn't know about the family financial problems, and so on.
Doyle wise, he's the one creating stakes for the story, both for NTR and non NTR fans. He's hideous as shit to look at don't get me wrong, but he is again the one shifting the blame so the MC isn't a sociopath.
Like, yeah, the author could just erase him, or make a character like Alexa from SexBot, another scapegoat for the fucked up house porn protagonist things, but then that wouldn't be Power Vacuum wouldn't it? It'd be a completely different house game, because these characters are never characters they are Masks, like a Commedia Dell'Arte, much like Hunter pretty much states after conquering so many other families, all mirrors to his own in some way.
Anyway, that's my two cents.