From here it gets progressively riskier. With no preboot mode to flash an rgn file a conventional cure isn't possible so if it becomes hard-bricked it will be non-recoverable. Another thing to try would be changing the HWID in the v3.01 GCD from 741 to 1036 so that the file might be recognised by the device. At your risk entirely if you wish to try that - I have no idea what will happen if it's flashed, it may boot up or it may die forever.

Here's the basic problem: we don't know if complete HWID 1036 fw was flashed or a hybrid mix of part original GVN53 and part foreign Edge 500.

Do you have any knowledge of what was done to it and importantly how it was done?

Can you let me have a copy of the regenerated GarminDevice.xml now on the device please? Best to upload it to a host site like ZippyShare and send me the link by Private Message.