Full agreement except last update gave me the strong impression that a path to make Katie and Jenna agree to share the MC with sis is potentially feasible as well, although to defuse the competitive streak between the sisters (and Katie's jealousy of course, but nothing new here) is going to be a mix of Herculean task and defusing a bomb. If this can be done, thanks to the latent polyamory plot hooks you listed that the sisters share with other girls, you have some definite potential to use such ties as bridges to build a partial harem ending that includes at least Katie, Jenna, Debbie, Lily, and Jolina.
I can't remember how Angel feels on the exclusivity topic. Bella admits jealousy but it seems mostly focused on her competition with Elaine; I assume in the end her attraction for MC and her wild streak can be channeled to make her accept sharing MC rather than losing him. Even with their ongoing sexual reawakening, Monica and Elaine seem too invested in their 'prim and proper' self-image to accept a polyamorous relationship, although they have been able to overcome the quasi-family hurdles of an affair with MC so who knows. Monica really doesn't seem the type that can accept the weirdness of sharing a lover with her daughters, and I don't care that much about keeping her due to her unattractive face and blue-balling tease attitude. She has the Zarah exception written all over but the only thing I want from Zarah is being able to kick her orcish face out of sight ASAP so who cares. Theresa is a hot youthful milf but the kind of crazy I find intolerable so I'm only in for the kink of bedding her a few times (provided it does not involve too much religious d/s weirdness). I am not a fan of BDSM so the less I see the other model, the better.
I am a big fan of harem endings so I eagerly welcome anything in this game that can bring me any closer to that ideal, such as being thankful for and relentlessly supporting any option that seems to make a girl more interested in threesomes, sharing, voyeurism, and/or g/g experimentation. Most of these girls seem so sexy and lovely that I cannot really turn down any opportunity for sex with them or making them happy in the brief term, even if I know that encouraging their implied expectations of an exclusive romantic relationship with love confessions is a disaster in the making and loads up the guilt meter. I can't stand breaking up with any of them so far, even if the alternative looks like kicking the can down the road. At least the partial harem perspective would allow me to keep most of the girls I fancy (Katie, Jenna, Debbie, Lily, Jolina, perhaps even Bella and/or Angel). On the other hand, keeping Elaine in a non-exclusive relationship sadly seems close to hopeless so far.