I am not a guitarist, so thanks for the explanation.
It does sound like a repetitive use injury and possibly a muscle imbalance and that you let it rest to heal. Please let me know if you have had any treatment (PT, meds, MD, etc.).
If I were you, I would limit reverse grip work to 1-3 exercises at the most. I would start very light and add weight gradually. Again, if anything causes pain (not muscle soreness) then stop immediately.
I would perform direct forearm work that would consist of wrist extension, pronation/supination and finger extension. If you are still playing the guitar daily, I would either do very little or no wrist/finger flexion exercises as it is already being trained on the guitar. For the finger extension you can use multiple rubber bands or check out ironmind.com for "expand your hand" bands. (For all forearm movements) I would start easy on the weight and do higher reps initially to help train the movement, increase blood flow and have an easier time detecting if a movement is likely to cause injury.