I'm actually not sure why there is so much debate on this subject. It was quite obvious to me whats happened so far if you understand that JK will very much reuse old plot devices in her work because... its easy and very much in line with her writing style:
Grindlewald was the master of the elder wand in FB1 since he's undefeated upto this point
Newt defeats grindlewald at the end of FB1 (he clearly restrains him with his spell) and is now the master of the wand (remember tina only summoned the wand after newt won so she's irrelevant to the discussion)
Newt is the only one actually able to defend against the protego diabolica spell in FB2 (watch closely, it incinerates the aurors while newt has no problem deflecting it out of his way) because as master of the elder wand the spell wont hurt him. Thesius only deflects the fire because a) he's alongside newt whos actually doing the work (unless i'm mistaken they're side by side for most of the scene) or b) because leta's 'love' saves him when he's briefly separated from newt (we do remember the whole love thing right? why else would leta's death change the fire like that and why would she mention "doing it to save him")
aaaaand thats where we are so far. Newt is pretty obviously the current master based on the above observations and we just hav2 wait and see what happens next.
The simplest solution would be grindlewald defeating newt in a later movie so hes back in charge of it for the 'Dumbleduel'
However i suspect JK is going to go for a rehash of her 'who's actually the master' plot device from harry potter - ie: Dumbledore will disarm newt at some point and will be the elder wand master (unbeknownst to grindlewald) so he will have the edge in the 'Dumbleduel' allowing him to beat grindlewald. This actually makes sence since dumbledore will use the same strategy to help harry beat voldemort decades later (Dude had to learn the strategy from somewhere, why not by using it to take down grindlewald first).
And that as they say.... is that
S.