Wow! What a misery! I'm so sorry you had this experience. I almost hate to tell you, but I've had several experiences of returning things to Amazon (at least a couple dozen) and they've all gone really well - except for one minor hiccup with one.
That one occasion was recently, when I dropped off a few returns at once to a UPS outlet, and got the confirmation/credit emails. Then a couple of weeks later, received an auto-generated email that said I hadn't returned the item yet and if I didn't return it within a time frame, I'd be charged for the item again.
I rang Amazon and they sorted it out right away, could see I'd returned the item, and made sure I wouldn't be charged. They sent a confirming email about this as soon as we got off the call.
One time I had to return something and all options required a printer, which I didn't have. I rang them and they said, no worries, keep it (it was a very small item) and they refunded me anyway.
Another time, I received a cellophane-wrapped pack of six journals that was supposed to contain four gel pens but the wrapper had been broken open and there was only one pen. I rang Amazon to ask if they could send three more pens. Instead, they gave me a gift card credit for the ENTIRE purchase!! And let me keep the books and pen. The gift card credit showed on my account within about 3 hours, just as the woman said would happen. It was automatically applied to my next purchase.
I've needed tech support once, and did get to speak to someone who was most helpful.
I'm telling you these things, not to rub your nose in it but to offer hope that maybe if you try again - see if fresh eyes will give the right help?? - perhaps you'll be successful. I don't know how or why they managed to mess this up so badly for you, but it's a significant amount of money and maybe worth one more shot.
I sure hope you can resolve this -and if you're just DONE and letting it go, I can certainly understand why.
I also understand why you feel betrayed and "like a divorced couple" as you described.
Praying you have no more issues with them.