In AL, you can screw around with any and every girl you like until you get to the beach house (which was C.4 iirc). If you continue to do so after that, you lose the happy endings for Missy and Megan - but can still get the bittersweet endings.
Where there is a hint of reconciliation at the ashes ceremony.
For the other main LI (Rena), you can pretty much screw around until almost the very end of the game and still get her happy ending -
I believe the BaDIK equivalent of that will be Sage.
You can also get a relatively good ending with a side girl in AL (the ex).
I believe this will likely also be available for Jade and for Quinn.
As for the two LIs you are specifically most concerned about, We
may have reached Jill's equivalent "beach house" point of no return as early as C.3. To get her happy solo ending we might have to go back and choose to not go meet Bella at the library.
Bella is happy that we are dating Jill and is more than fine listening to our tale of M&J, so her happy solo ending shouldn't be closed yet. I believe her point of no return will come if/when MC inserts his D into Jill. If we do that, there is no coming back to Bella, we'll likely have to let Jill go before that point to get a happy Bella (IMO).
These are just my own working theories though, so don't go rely on it or anything.