There would certainly be much greater legal issues with deciding that B-4 = Data than with deciding that post-Genesis Spock = pre-Genesis Spock. From the layman's point of view, at least.
Not only are B-4 and Data physically distinct, we don't even know for sure that they would be "related". Data is the "legitimate son" of Soong, as much as Lore is - but B-4 could be an unrelated fake created by the Romulans for all we know, another component in their original plan of attempting to replace key Starfleet personnel with copies.
The deciders would probably err on the side of caution here, as they wouldn't be motivated to do otherwise. Reaccepting Spock would mean accepting a person who had performed world-saving heroics both before and after the confusing swap, while reaccepting Data in the B-4 guise would mean welcoming a traitor who hasn't yet done much to redeem himself.