Solved it. Both Telemetry and RC. I was having two different problems on two different radios.
For the benefit of others who might use the pre-compiled ardupilot-blue, I’ll broadcast my noob mistakes here.
The 3DR Sik V2 telemetry radios were fine. The BeagleBone and ArduPilot software were fine, the problem was that I was using UT0 rather than UT1 on the BBBL (whose pins are preconfigured for telemetry in ArduPilot-Blue). The command switch that ultimately worked was:
and for GPS (which had stopped working)
Then, on my RC link problem, I’d failed to successfully change the output from my ezUHF RX to PPM on Pin 1 using the Immersion Tools software and USB connection. Seems I hadn’t hit the “Upload” after toggling my choice.