I'm sorry if this comes across as harsh, but you're kind of letting them do this.
You can't be a demanding (reasonably) and upset customer and then fork over $400 for return shipping on a defective product. You showed your hand that you're patient and willing to pay.
It sounds like you're dealing with assholes. I don't really know what you can do at this point other than contact the consumer fraud bureau and prepare your documents to defend your case. That might light enough of a fire under them to do something. If you do, demand that shipping back.
https://www.ftc.gov/about-ftc/bureau...mer-protection