which is how, in theory (because I don't recall either, been too long), you can keep being on Jill path and attend Bianca's diner while being a chick/neutral but ending with a permanent DIK affinity
No, I don't think it's possible. The trigger for the party to happen is whether Jill feels Tremolo is right for her or not (basically, checks if Tremolo is Neutral/CHICK or DIK). But this trigger is too close to the end of the 2nd season and if you're not deep into DIK or CHICK affinities you'll stuck with Neutral perma affinity, if I remember correctly.
Before you get perma affinity you have to make 15 major choices and you can make
only 4 choices of the opposite affinity and still get DIK or CHICK perma affinities. But at this point (before the trigger happens) you have to make 5 or even more CHICK affinity choices to keep non-perma neutral affinity.
This, for example, works for Maya/Josy where you need be non-DIK affinity to get on their route. But because this trigger happens early in the game you can temporarily become Neutral affinity (like 2 or 3 CHICK major choices) and then go full DIK major choice and get DIK perma affinity.