I wouldn't say that there isn't any real evidence. That and the avatar being a shard is given as a guess. Sure, you can say that the only reason why they don't give real Golbez any voiced lines the 1st time you see them with Durante is to hammer home story wise that the Golbez we have been dealing with isn't the real one. Yet if that's the only reason then why not give them a different voice? Why then later have them imposed ontop of the WoL and shoot that scene from a perspective where it comes off as Durante is somehow directly speaking to the WoL and the real Golbez? That he could be talking to both of them even after the real Golbez steps forward. You can also say that the only reason why they didn't want to show us the face of real Golbez is to save themselves from having to render a really big Ardbert head. But with them not showing one it gives more weight to the theory as the only real explanation would be that Durante decapitated Golbez and the head is still inside the helmet.

To me the only reason why they finally have us shake the only other character many pegged for our 13th shard is to put that theory to rest with there no being any what I'm calling a soul recognition glow. As we didn't get any with Zero unlike the two times we touch Ardbert. Now you could go and say that light only showed up due to the light of the Lightwardens acting up but then why linger on the fistbump if it's only meant to be a fistbump. Course with the avatar you could say that the reason there is no soul recognition spark happening is it would get annoying real fast any time we go into avatar mode. Or that the avatar itself is too corrupted for that spark to show. But until they show the WoL in a cinematic as a reaper and thus make reaper a cannon job they picked up I don't think we should give the avatar being the WoL's 13th shard more weight than real Golbez.