Miami could still trade down, but a few things will need to happen first. Obviously, the Falcons would have to stay put and not draft QB4. If this happens, they will likely select Pitts or Sewell, followed by the Bengals selecting whichever of the two the Falcons didn't.
From there, Miami could trade back to #9 with the Broncos, but since there isn't that pressing of a need to jump (unless the Broncos fear the Lions might draft QB4, or that another team will trade with Miami instead and leapfrog them) we will have to take less in return while accepting the fact we will likely walk away with WR3 (assuming the Lions take Chase and the Panthers take Smith/Waddle).
That, or the Dolphins could fall way down to #17 (Raiders), #19 (Redskins), or #20 (Bears). They would obviously get way more compensation for both this draft and future drafts (probably a 2nd or 3rd this year and two future 1sts), but that would see them having to accept a great player rather than an elite player.