It seems implausible that his stumbling was a result of a back injury.
I came in to post about this. His stumbling because of his back and falling was completely natural for a lower back injury if you've ever had one. I have. My back completely gave and I just fell to the ground. I've seen it happen in other manual laborers. Back just gives and they're on the ground.
Happens all the time. It's highly unlikely the mouth breathers of the NFLPA aren't aware of this. I'd trust them as far as I trust the owners.
That's all I have to say about this aspect.
It is highly unlikely they got to the independent neurologist and convinced him he was okay to play. The back thing and its plausibility is exactly why Tua was cleared to play.