Tough call... it really depends on how believable you want it to be. There is such a thing as too realistic, especially in porn! If one of the girls realises she was possessed by the MC, realistically she should be terrified beyond any capacity for rational thought. Keeping things relatively light hearted was a good call.
If you want to give it more impact, since the MC and the girls are all on good terms, they might try to approach him and ask "how" and above all "why" does he do that. The MC may have the chance to admit his errors and face his guilt, saying that even he had little idea of what he was doing and that never-ever meant to do any harm, etc.
It is a lot of drama and not everyone likes drama. We write porn and our sole goal should be give our audiance a boner (or making them wet, we are for equal opportunities).
Yeah centering it on the relationship with the heroine in question is probably the best way. If they know exactly what's going on anything other than terror just doesn't make sense. They'd probably intimate they know something is going on and ask them to stop even if they don't know exactly what's happening. This would remind the MC that "oh yeah they are dealing with another person here, not a toy." The MC might admit they did something wrong, and say they never meant to hurt the heroine. It's a fun idea but like you say doesn't fit the tone of the game well.
It would be perhaps fun if the possessed was forced into the possessor's body. That would open a whole new realm of comedic and erotic possibilities
"I'll show you how to fuck me right."
I'm getting carried away (which is good).
Oh yeah that would be a darn good time. Sadly it would be a bit outside the theme of the game. There was a very specific reason that you don't get with the girls you possess. They are all looking for something other than the MC, and the game all but says that trying too hard to be what you aren't can be harmful. Also to sort of invert how these games normally go. Also the structure of the game basically means that if it happens it has to be it's own path, just too many related scenes.
But if I was going to do it, I'd probably do it like something about the possessed getting forced into the possessor's body makes them either horny, or slightly addicted to using the stone like the MC is. This is both because Alice/Diane would never cheat on their significant others and to prevent May from beating the crap out of you.
So if that's how it's happening, I'd still say things wouldn't unroll into just straight sex. Diane would be more interested in seeing MAGIC! right in front of her, and would do a lot of not quite sexual experiments to see how things work. Alice would probably take on a big sister sub type role. She'd see the whole possessing someone else as a cry for help and figure introducing you to just how good surrender can feel would help. You still aren't getting near Max. May would for a moment be annoyed at having your body and excited to try taking control during love making. Then she'd realize what she enjoys about sex is the contest during the act, and try to get you to act more dominant rather than just rolling over.
It's a fun thought experiment. Thanks for getting carried away!