It seems to me that you are claiming the Patriots can shelve Gronkowski indefinitely due to the fact that he missed an entire season. You have stated "Its's up to the NEP if they want to remove him from the reserve/retired list. Gronk can say he wants to come back all he wants. He is powerless." If I am misunderstanding your thesis, please feel free to set me straight but it sure sounds to me that you think the Patriots could shelve him indefinitely.
My contention is that if Gronk reports to the team at the start of training camp, the Patriots will have to pay him by week 1 or unload him. I also contend the same applies if he reports during the season prior to November 30 (give or take a couple weeks grace period).
I am not going to argue the bureaucracy involved in returning. The Patriots can stall, but only for a very short period of time. They cannot shelve him indefinitely. They would have to decide by week 1 whether to piss or get off the pot.