--that compression issues in the back were impinging on nerves that track down the legs/thighs and producing symptoms.

If one has kept up one's B12 ingestion, it is highly unlikely that this new pain/sensory disruption is from depletion of B12 stores, but more likley from biomechanical issues.

Not to say that B12 isn't important for neuroprotection and rebuilding, but it's not the only issue in nerve conditions, and sometimes not the primary one.