Many thanks to everybody who responded on this forum and a couple of others for the ideas and suggestions ... the crisis has been averted. Turns out my USB OTG cable was the problem. Today I purchased a new one, plugged it in and bingo ... life is good.
I've also been able to win the battle with the somewhat clunky Bluetooth functionality of the Garmin 810. I'm now able to sync routes to the Garmin that I've created and downloaded from Garmin Connect. And, if for some reason both of those options are not viable, the micro SD card from the 810 can be transferred, and written directly to when inserted into the tablet's SD card drawer. That's an option that would quickly become tedious but its reassuring to know it's there in the worst-case scenario.