I've made a flash kit for you from the "Montana 590.gcd" in the ZIP file you linked in Post #11. If you decide to go ahead just follow this:
  • Get a suitable small clean SD card formatted in FAT16 or FAT32;
  • From behind the Spoiler, download the ZIP file and extract it directly to your SD card, you'll then see Garmin folder and fw_all.bin in the root;
  • Ensure the montana is fully OFF, not in standby. Force it off by holding the power button down if necessary;
  • Insert the SD card;
  • Power on the montanna (if battery is less than 75% charged, connect it to an external power supply [NOT a PC USB]) to power it on;
  • "Loader" or "Software Loading" or similar message should appear on screen as it starts up;
  • Allow it to finish and let it reboot, do not let it lose power during the flash.

Hopefully the downgrade will invoke a hard-reset after it boots up following the flash, if it does so but you find that the screen is still unresponsive then the problem is elsewhere. Be prepared that the flashing effort may even kill it - if the chip is already compromised as i suspect.
Spoiler: Flash Kit
[Only registered and activated users can see links. ]


If a hard-reset is not invoked by V5.90 then an even lower version might need to be loaded, however it gets riskier the lower you downgrade that the device might brick.