That further info cements my feeling that it's only badly 'soft-bricked' rather than suffering some random hardware failure, such as a failing flash chip.

Leaving the nuvi without the battery connected for an extended period of time will also drain all residual current from its circuits - sometimes just that can also help by forcing a factory reset.
Aye i'll not be doing the Glasgow Kisses then. Merry Christmas Rusty and do dheagh shlainte back to ya (no single Malts for me, i'm a dedicated Dewar's man).