Give it a shot and see how he acts unbanned, you can guys have the power to instantly ban him if needed, im almost certain he has no reason to make accounts / borrow if he is unbanned lets be honest here.
We have no basis upon which to believe that. His ban stays and the entire C&C:Online staff (Revora and GameReplays) agrees on that. If you want to stop him from playing with others through dupes, you might as well convince the clique he almost always plays 2v2s with to refuse playing with him for once, so he'll have nobody to do it with.
Then here is a suggestion from a neutral party:
Why not getting a new staff member who has absolutly no connections with neither side before hand (as far that is possible ofc) and let him watch over GZ for a set time span, including ingame behavior and behavior during tournaments which the new stuff member will suprivise. (Ofc the old staff members are regulating the usual buisness)
Then both, GZ and the GR Team have someone between who doesnt have a prestressed relation with each side and act acording to his own ruling about situations which might accur.