I finally got appointments squared with Kaiser Ortho. I had a 2nd x-ray last Friday - day 14 after the break - to verify that I hadn't displaced the break. Everything was good. The doc gave me the all clear to ride indoors on the trainer and to start working on range of motion. She said that if I get sharp pains AND it's sore later that day, I tried too hard and did more damage. Final appt is Apr 8th, a few days before my next MTB race
My brain took all that amazing medical advice and heard "you'll be fine riding outside". So that Saturday - day 15 - I did a short 2hr ride outside and was fine. It hurt a little when accidentally pulling on the bars and not at all when pushing on them for descents, braking, or potholes. No harm done. I rode outside again Wed. Because it was a night ride, I stupidly wore a base-layer undershirt. It took me 10min to get that undershirt off. But that did more for my range of motion than any previous exercise

. I'll probably do 4-6hrs outside this weekend and will ride the MTB again the following weekend.
The potentially-cracked rib has finally stopped bothering my sleep. It's still annoying, but only when getting in and out of bed.
Also, I've now done 10 sessions on an indoor trainer. I absolutely hate it and can't wait to be done with it. It's so sweaty. The RPE is higher. It's all too static. Ugh. Thankfully only 2-3 more.