I have but I never accepted the deal and I have no way of knowing if a broken promise is even possible. I can't know for sure because a game is a series of arbitrary design decisions, it's not real life.
Based on what I've seen so far I doubt you'll be "forced" to share Bella (or any other girl), and frankly even if the Dev does implement an Netorare route where Gramps fucks Bella there is nothing stopping you from reloading back to where it branches.
Regardless of your choices I'd strongly recommend saving at branch points regardless (there are lots of save slots, so no reason not to use them), there is already a "bugged" path where submitting to Vanessa during her date at the motel blocks certain content. and there are several points where certain scenes stop being repeatable so having archived saves is handy if you want to revisit any of them.
