Hi Sam,
Sending this from a airport somewhere in the American mid-west, so will be brief (and not read your other posts yet!).
No, you cannot delete a shot from the DistoX memory as far as a I am aware, and no you cannot download data twice. The reasons for this is that the DistoX transmits each shot to the PocketTopo application, and when it has done so it deletes that shot from it's memory. This is how it knows what to download, by only doing it once. PocketTopo has been written to be as fail safe as software can be. If it does crash, then I believe it leaves a copy of the data in a temporary data file, plus a second one on the memory card of your PDA if it has one. So you should be able to recover from a crash without data loss.
Footleg