I am disappointed that no JRiver folks would respond to my plea for help after over a month and two bumps of the thread. ANY kind of response would have been appreciated even "sorry, you are on your own because we have no idea, are too busy, etc". Personally, I feel frustrated left hanging in the wind when posting on a forum for resolving technical problems.
Since I was constrained on disc space and couldn't try this before, I ended up buying a new external disc drive and created a folder on an SSD external drive and using "send to" sent the 2600+ music files over there. Then I connected my Pixel to the computer and did a copy and paste of all the files from the SSD drive to the music folder on the Pixel. I just checked and now all the files are there.
With more steps than using the Sync Handheld function in MC, much head scratching, and $250 in new hardware, now my issue is resolved inelegantly - with zero technical assistance from JRiver it was the only work-around I could figure out. Updating the thread in case someone else has a similar issue and is looking for a cumbersome workaround.