Consider that there might not even be negative feedback but that they just made that up to get rid of you for some other reason they are not willing to divulge.
As far as your approach, telling a company you have no interest in them doesn't increase their incentive to basically do you a favor. Most companies will take "I'm not interested in working for you" as a bit of an insult to the company, so leadership isn't likely to go out of their way to help you once you've said that (regardless of the other circumstances you've described). It comes across as sour grapes at this point.
It sucks, but I think you need to move on from this one.
Let it go. You've stated elsewhere you nearly damaged a friendship over this. Don't go full Don Quixote.