It seems pretty clear to me that Grindlewald has the wand the entire time. It is simply masked with a charm to alter it's appearance. Why do people see this as a difficult thing to grasp, when Grindlewald himself was masked as Graves. I mean seriously, which do you think is harder to change the appearance of - a human being or a piece of wood?
Watch as he battles ALL the other aurors. You'll notice an interesting fact, even with dozens of aurors, enough to destroy the largest obscurious in record.... NOT A SINGLE ATTACK penetrated Grindlewald's defenses. No wand was able to defeat him.
And the writers did take this into account. How was Grindlewald defeated? Not by a wand, but by one of Newt Scamander's creatures, the " Swooping Evil".
It was very very clear to me, he was wielding the wand. Go back and look at all the wand duels and battles. Not a single one got past him. And I also wager that Albus Dumbledore will defeat Grindlewald, and take ownership of the wand, not by dueling, nor by trickery or other means. I suspect, Dumbledore will defeat Grindlewald with logic, reasoning, and love. And Grindlewald will hand over the wand, seeing that Dumbledore is in fact correct. And that Dumbledore will have won Grindlewald toward his view.