Sorry if I couldn't answer early. I know hidden fields are not best practice since they are easily tempered with. Proofs could be moved from standard to user nested resources but doing this would need using a hidden field to set standard_id field to the proof instance. I don't know what it could be more critical, whether tampering with a proof standard_id or with its user_id, I believe changing the standard_id is less critical.
Concerning your second comment, I don't understand. Did you try pulling my changes and the specs aren't passing?