Sorry BBC, but the last PT I made here with permanent Neutral affinity disagrees.
Quinn is the most disrespected character when it comes to choices and affinities.
You can spend half the game avoiding her, then decide to smoke a joint and fuck her on the roof. Or spend even more time avoiding her, and decide to help her out in the middle of the street, which also leads to sex. Or avoid her completely for the entire game, and have a threesome with her and Sage. At no time the affinity was taken into account.
While "creating a relationship beyond sex", which has happened to her, it also doesn't need DIK choices or even DIK affinity for her to end up being an option for the "others" in the final decision.
The only thing that ends the MC's relationship with her, if not for your lack of self-initiative with Quinn, is continuing a relationship with Jill.
On the contrary, Quinn might be more linked to affinity/status and choices than any other character - starting from episode 1. You need to be Huge DIK to get 100% completion of the scene with Quinn and Camila in the shower room. Similarly in episode 4 you need DIK affinity to unlock the whole scene with her in her bedroom when MC stays over at Sage's.
It's true that smoking weed on the roof isn't dependent on anything else - but it's the unofficial start of Quinn's route and becomes a key condition of unlocking each subsequent step. Helping her in episode 6, for example, can't be done if you've been avoiding her; MC needs to have had that roof scene with her
and also have DIK affinity. If your playthrough was on neutral it must have been after that moment. The hangout with her in episode 7 is also dependent on having helped her in 6; key scenes in episode 8 are linked back to 6 etc.
You're correct that you can unlock the threesome with Sage and Quinn even if you've been avoiding Quinn completely and have at least neutral affinity - however the build up scenes are different, take account of your past choices and depend on whether Quinn likes MC or not. If you haven't been on her route, she'll say "I don't even like this guy", and there's no scene afterwards with her. She will refuse point black to do anything if you're CHICK.
There are inconsistencies on all the girls' routes related to affinity and choices etc, particularly if a player is doing affinity swings and making contradictory choices, but I think Quinn's path is holding up fairly consistently so far.