Thanks Neil!, if i can unbrick my 2797, I owe you the 95% of the device!! I tell you what I'll do. Since this 2797 try to boot like a 3597, i supose that the white line is the "Garmin" logo mapped in a different screen (7 inch vs 5 inch and a different tech) , so if it's true then the point in the screen to pressing to enter in preboot mode maybe is in other place. . So I'll pressing with a plastic point in every place in the screen, one time for each plug/unplug the usb cable (damn, why I bought a screen so big?). I locked the enter key of my keyboard with focus in the OK button with the Updater app running with the rgn in your post. If the nuvi enter in preboot, even a few hundredths of a second, then the updater will perform with the flash.
Obviously if I can unbrick I will post the details!!