http://ardupilot.org/dev/docs/skyviper.html
Search for stm32 on there; several methods are given.
I’ve tried all the methods in that link (SD card, sonix board, web AP) and they don’t work. You mentioned a method involving soldering a Serial to USB adapter directly to the STM32 board and flashing it that way, which I cannot find any documentation on how to make that connection and force the flash.
That soldering method is for bootstrapping the Sonix board, not the STM32… :\ sorry you’re having such issues. These things were “mostly” reliable right out of the box. Hate there were a few bad eggs. The worst thing is, you might have gotten one that was returned and Toys R Us just put it back on the shelf. There’s a reason they are out of business.