His contract means nothing until such time he is on the senior list via one of the methods the AFL has available for clubs to aquire players. The 5 year deal is purely a statement of intent, not one that is valid now. He can be training at the club now because the AFL allow this for eligible father sons.
GWS absolutely can list him as one of the 17 year olds because they can't pick these 17 year olds themselves, they MUST trade them to another club. His contract with Melbourne means nothing because the AFL won't recognise the contract until he is on a senior list. The contract doesn't mean he gets to bypass the draft, he still needs to register for it as that is how Melbourne will select him. Even Adelaide were hoping to use this rule to get Viney by getting GWS to list him as a player so Adelaide could trade with GWS and get him before Melbourne could take him with their father son selection but they were undone by the fact that GWS doesn't get to pick the 17 year olds they list, it is done during the negotiation process with the club.
Melboune have every right to tell the player that they definitely will take a player under the father son process, however this still doesn't mean he's on the senior list as he's not, he can't play a game with them, he can only train with them which he can do anyway.
Stop being so pig headed and just admit you are wrong.