I'd say Jill's is the "weird" relationship, in the end the relationship with Sage is the counterpart to Maya and Josy's relationship of just friendship.
Bella is married, so it's realistic that if Mc doesn't show interest that's the end of it. but Jill? does it make sense that she wouldn't involve him in a matter that completely affects him?
on the other hand, about the idea of being able to get back into a romantic relationship: theoretically it's possible but wouldn't it be a shortcut? on the one hand, mc has to live a whole growing relationship where he goes from recreational sex to restorative cuddles, on the other hand, it's enough for him to choose Sage as his girlfriend at some point?
In my opinion, Jill's path (blackmail aside) makes sense. If the MC shows interest in her, she'll pick up on it and reciprocate; if the MC goes through the motions, she'll eventually assume he just wants to be friends and dial back the interaction.
With Bella, the MC seems to act like there's this smoldering mutual passion between them purely on the grounds that they kissed while he was drunk. Even when he breaks things off, it's on he rationale that the two of them just wouldn't be able to control themselves if they were in the same room for a couple hours. Sorry, not seeing it.
As for being a shortcut, not really. You'll still have to help Sage sort out her problems and spend lots of time with her to get the second opportunity; based on what DPC says about Episode 7 it seems likely there will be some trade-offs in that. Plus, even if you do become her boyfriend without FWB, you'll still have to face the music for any other relationships you have at that point.
Different path, but basically the same destination.
No, i know there will be consequences if MC keeps FWB with Sage when he's in a relationship with someone else, my MC is on Bella route but i helped Sage with her guitar lessons and i'm on her route too, now i'm getting all of her sex scenes so i was asking if this is going to have consequences later on? because i want to be able to cut things off with Sage without hurting the relationship with Bella.
Well, you can always go back and not become FWB; either keep the guitar lessons professional in Episodes 2 and 3, or don't drink with Sage during the HOTs movie night in Episode 4.
If you just can't pass up Sage's womanly charms, I don't
think it will ultimately stop you from pursuing Bella but we'll have to see what happens to know for sure.