There is nothing wrong with assuming the best of people. I try to do that myself and I find that assuming the best saves me a lot of psychic energy.
However I have really good bs detectors. If there is a whiff, a hint of maliciousness, of drama for drama's sake, or just crazy-making tendencies, I get skeptical. I rarely give second chances at this point because something is not right. I may not know what exactly but I trust my instincts. They have rarely failed me in seeing and getting myself out of potentially bad situations with scary people.
This couple has done more than give you a hint of maliciousness or drama seeking. They've waved the red flag under your nose for some time. Use this experience to develop your own bs detector. It will serve you well.
Also I applaud your willingness to talk and listen here. That takes guts. Good on you. I was skeptical you would be able to do that. I like being wrong sometimes!