1) Then why only using it in Block D? And on a moment all of the weak ones were pretty much already beaten?
2) That option is what I already considered in my post and doesn't make any sense. After this round the three fighters of the other blocks are waiting. If you couldn't even survive the fighters of the first round without help, you have nothing to search there. That would be plain suicide. Of course if this is a two-man team, they might simply try it on them too, however seeing how the story is progressing, this seems highly unlikely.
3) Refer to #2, the 2nd round is far more dangerous. On top of that in Dressrosa losing is 'not done'. All the losers are discarded. Of course most people probably don't realize that, but someone who can pull that off is not some background character. So whether winning or losing, both is dangerous.
Most logical reasoning remains that one left standing is doing it.