I'm surprised your PT wants you to give up riding. Does she just want you to lay off riding for a while, or really want you to give it up? Ask her what she meant?

We often encourage our patients to take up riding, because it's such a good sport for folks who've been injured. But then our clinic owner is a huge biking goddess...

If I had a patient with a situation similar to yours I might ask her to try switching to a recumbent bike for a while to allow everything a chance to heal. Not a flat-out supine recumbent, something more like a RANS or a BikeE, Rush, TourEasy, where your spine is more upright with a bit of a lean back and a fully supported spine. But only if there were no lingering balance impairments from the injuries. If someone's balance and proprioception haven't recovered yet I don't encourage biking.

I would also reccommend my patient take some of the really good bike safety and handling classes we have around here, not because my patient doesn't know how to ride, but because after a brachial plexus injury a person's body needs to relearn some skills that have been taken for granted in the past, and a class with an instructor who can watch a person's body is a great place to retrain.