Mike, I think I stumbled upon the answer fairly quickly and avoided a lot of hair pulling in the process.
The "secret" seems to be to pair everything to the Garmin and delete any pairings between the other devices.
- Sena 10C Bluetooth headset paired to Garmin GPS.
- The cell phone paired to the Garmin GPS.
- All other pairings, like the cell phone to the Bluetooth headset are deleted.
This way, if I don't turn things on in the "right" order, they still make the right pairings. When I had the pairing still going between the Sena 10C and the smartphone, they would connect first and block the Garmin GPS from pairing with either.
Chris