IMG file have a bit set that determines whether Basecamp can 'see' the map.[This is to prevent maps created thru Basecamp being seen double]. There is a tool called Javawa Device Manager that will change the visibility - worth trying.
There are technical reasons why a single Garmin map of the US cannot be made. Garmin devices vary but are limited to a few thousand map segments and, for the maps I make from USGS, each state is a few hundred although it depends on the level of detail, too.
I'm not sure exactly what you mean - but my MAPC2MAPC will open a 'Custom Map' KMZ (tiled raster images) as a single calibrated image. You can then paint GPX tracks onto the image in various styles/colours and save the result as another KMZ. It won't do arrows, though.