Unfortunately, I think you might need help from Prusa. If the cpu won’t come up with the current firmware load, and factory reset doesn’t help, then the only way to install new firmware is the way they do it in the factory – via JTAG, which is a serial interface on the board that lets you connect a JTAG programmer that can reload the flash without help from the CPU.
This means you need a JTAG interface card and programming software for your PC, and the knowledge of how to plug it in and do that (not impossible 🙂). If you’re lucky, you might find a tiny connector on the card, only 5 or 6 pins, where you connect it. If there is no connector, you will have to solder something on, assuming you can even find where it goes. if there’s a Prusa rep near you, I would at least ask them if they can help with info, or they might b able to reload your card for you or swap your card for one that has a good load on it. Good luck!
Collaborate with makers in the community to develop your device idea.