There was even some speculation that BB and Moria were in cahoots during Marineford- BB would steal WB's devilfruit, and Moriah would get WB's shadow.
You cannot hav the shadow of the person after they are dead. Cindry's shadow returned to Margret who was alive, same goes with a lot of the other users.
So if BB indeed worked with Moriah, it would've to be for the corpse and now the shadow. Moria would have to dig it out of WB's grave for that and we know that is unlikely since he was attacked by Mingo.
But there's the big reason he's not a zombie: he walks around under the sun. Unless Oda is somehow retconning Moriah's ability or making some special exception for Weevil, it doesn't seem possible. Also WB seemed to die in a relatively natural way, as opposed to the top-down dissolving that zombies exposed to light would.
No, Weevil isnt a zombie. But he could have parts from WB's body, like arms or torso.That isnt as strange considering how disproportionate his body is and also all the scars are. He could have multiple shadows in him, specially of WB pirates who were taken down by him.
I dont see how Moriah's ability with Weevil would be a retconn either. Moriah can swap his body with a shadow at will and also extract other people's shadow. It wouldnt be a surprise if Moriah can actually hide himself inside someone's shadow. If Moriah is doing with with Weevil, that'll make Weevil not only more powerful, it might explain his quick success too. Moriah is not weak inspite of having lost to Luffy, but his DF ability would work best from the shadows (pun intended). If Moriah is actually hiding inside Weevil's shadow, he could extract Weevil's opponent's shadow weakning'em for a quick and easy blow.
Also, i consider it this way:
Moriah entered new world, lost half of his crew to Kaido.. his lesson learnt: build an army of Zombies so you get a loyal, painless followers.. while the zombies dont feel the pain, Moriah doesnt feel the pain of losing more crewmembers...
From Doflamingo's attack, Moriah realized that the WG are wanting to hunt him down too. Considering how he cannot even trust his counterparts, its best to hide completely and exist. Moriah might be seeking refuge in Weevil's shadow...
Coming back to the Kaido argument, Moriah stealing his shadow will clearly not be enough to take down Kaido... but may he the first blood you need to draw in taking him down...