I still think the key difference is that Quinn is optional. You can choose to get involved with her if you want (and meet her criteria), but unlike the actual LIs you can also just skip her personal problems and interact with her only as an antagonist.
You're right that in theory DPC could always add more LIs later, but I doubt he will do so. If the current 5 LIs were added as the MC uncovered their paths, it would make sense to keep adding new ones. But the LIs were presented all at once at the end of Episode 1, even though Bella had shown no interest in the MC and Jill was barely in the game. Even Maya's status had just been thrown up in the air thanks to cliffhanger.
For whatever reason, those 5 were singled out for the end of episode status report. To me, that suggests there's more to the LIs than the other girls the MC fool around with repeatedly. It might be as simple as them being the only ones with proper endings, but I think it goes beyond that.
Either way, though, I don't think it really matters if Quinn is ever made a formal LI. Her importance to the story is obvious, she has her own version of all the LI activities, and it's not like being a formal LI in a DPC guarantees the girl a happy ending.
mad
As far as Zoey is concerned, right now she has no connection to current events at all. I'll wait to see how (and if) she's reintroduced before I judge her romance status.