WE Can detect corruption in Nuvi Normally by looking at garmindevice.xml but of course you cannot get it because it is not possible to get files from Nuvi.
What I wonder is how Garmin Express is able to see your device. (Maybe it uses some other protocol)
Try to connect to Garmin Express with the SD Card Removed. Can you do updates such as Voices etc from Garmin Express ? (With the SD Removed)
Your Nuvi is Actually Not an MTP device. (It should be in Mass Storage Mode Permanently Anyway)
Can you select different voices ? (Voices are files in internal hard drive)
Can you see the SD Card as a drive in the Computer when it is in the Nuvi ?
Try to clean the USB connector on the Nuvi with some contact cleaner or similar.
I'm leaning towards a hardware fault because GarminCure3 is working very well for many thousands of users.