Super cool of your parents to support you like that, not cool of them to control you, most parents like this miss the memo that kids need to actually start making decisions for themselves.
By sunk cost fallacy, I assume you mean about the money. Giving them all the money back at once is an interesting idea, how would you feel if a child of yours did that? How would you feel if they were up front about their new situation?
You know your parents and your situation better than any of us, use empathy and put yourself in their shoes and make the best decision you can.
That's pretty concerning, my mother went through a similar relationship early in her life, he was older than my grandparents.
You do you, but anecdotally, for her, the relationship may have started out okay, but they were never equal. He treated her like a daughter that he fucked every once in a while. Power dynamics in relationships are extra screwy when the age gap is more than 20 years. Our holiday dinners at home were always very awkward.
Their relationship lasted about 20 years, imo it should never have started in the first place. She remarried afterwards to someone her age and finally found some companionship instead of stewardship, that relationship lasted much longer