Propostioning Bella doesn't mean they're as close as friends. They're work colleagues and probably close enough in that sense for Stephen to think it was a good idea to proposition her since he could potentially rely on the belief in "professional courtesy" that she wouldn't mention it to other people should she approve or reject. We also don't have any indication that she Jade are or were friends at any point.
I would suggest that most don't share intimate details of their love life with mere colleagues, let alone bring them into the marital bedroom. That level of trust implies a deeper relationship to me (i.e. friends+), particularly given Bella's reserved demeanor. Surely mere colleagues wouldn't expect Bella to be up for it, she hardly screams swinger right? I don't think professional courtesy covers swinging either...
As you mentioned in a previous post, they were still close enough for Stephen & Jade to invite Bella to dinner in Ep 1, so hadn't fallen out yet:
They were chummy in Ep1, things went pear shaped after that. I imagine the proposition happened at said dinner, which fits the timeline of the events being discussed.
But what is it specifically about staring at Jill that made her do that?
We don't know, we don't need to know for the scene to work. Some interpret the looks to hint at underlying sexual tension, I don't see any of that. Bella is cross, Jill is concerned then offended. Anything beyond that is baseless speculation imo. I've offered alternative explanations for why they may have argued which are as plausible, if not more so, than the Bella/Jill thing, but they are also conjecture.
Jill wants to leave the room because they're arguing, it neither offers nor needs an explanation beyond that.
It does seem somewhat obvious, yes, and DPC generally doesn't do obvious, so perhaps this is a wild goose chase, but as of right now it does feel like it points in this direction. If it indeed turns out to be something else, then it would have to be something that connects all these dots in as clear a manner as this current theory, but right now I simply can't fathom what that could be.
I don't think all the elements you're raising accurately reflect what's portrayed in the game, and like ename144 don't think all of them connect.
I didn't expect you to waiver in your views of this scene though, as you're clearly wedded to it. I think it's useful to point out it's not the only plausible interpretation however.
I consider the entire concept of the Bella/Jill thing as completely at odds with their characters as presented so far & would be astounded if that were the case.