OP, I think you handled this absolutely fine. I think you were being scammed right from the start, particularly because of the 'excited' texts the salesman sent you (which to my mind were really unprofessional). The GM's manner was the icing on the cake, and really, they deserve to lose custom. Good luck with the new dealership!
By way of comparison, DH and I had our windows replaced a few years ago. We had a quote from one company, and then a salesman from a second company came round. He claimed to be able to match the initial quote, and when we told him what it was, he was visibly taken aback, it was clearly much lower than he had expected. Nevertheless, he called his boss straight away and they agreed to honour his word; we got the windows for the low price.
If we ever need more work, we will definitely consider them, because they behaved professionally.