I’ve just watched this a few times on MLB.com. You need a subscription; i don’t think they included the play in the free videos.
First, i completely disagree with denquixote that we can’t be certain that the fielder dropped the ball. It’s totally clear from the replays that, while the ball goes into the glove cleanly, it drops out and lands on the ground as he’s sliding, and before has has control over it. Neither set of commentators was in any doubt at all about this, and the Pittsburgh commentators were, if possible, even more certain than the Miami commentators that it was no catch.
I took a few screen grabs of the end of the play. Here are four, in order:
1 2 3 4
Clearly not a catch, IMO, and i believe that even an NFL umpire, with the league’s “incontrovertible evidence” requirement, would have ruled it “no catch.”
So, where should the runner be placed? It’s true, as the OP says, that we don’t get a really good look at everything the runner did. I was incredibly surprised that they didn’t give us another angle on the runner’s actions during the play.
I did, however, take some screen grabs of what we did see, and in my opinion the runner was clearly headed for third base, even while hesitating to see whether the ball would be caught. The last instant that we see him, he is about half-way between the second and third base, and is still leaning and moving in the direction of third. We don’t see exactly when he turns around and heads back to second, but i believe that he was clearly heading for third for much of the play, and he was also close enough to third to have easily made the bag if he had known that the catch was dropped.
Here are some screen grabs of the runner’s progress while the ball was in the air. The last one shows the final glimpse we get of his head, and that’s all we were allowed to see.
1 2 3 4 5
I think the umpires should have put him on third.
All images can be seen in a single gallery here.