Just a bit of experience
I once had a similar problem with a CH sw and my 1200, .device.xml was corrupted after updating it with Webupdater with US sw, but nuvi was working fortunately.
The only way to restore .xml was to reflash it to initial alternative CH firmware, then reflash to an old US sw, resetting it at every step: Dasilvarsa wrote the same above
I think I flashed it at least 30 times before finding a solution lol.
In your case we should know which kind of damage has been produced in nuvi, I guess you dont have nuvi NV backup