Yeah- I've seen plenty of scifi shows where their memories were wiped on purpose but they screwed up in some way and then wound up remembering and had to wipe their memories all over again. Some sort of "I did this, don't mess with it" message from Jack to Jack would have made a lot of sense.

and the sand? No clue. Was it what was left of Adam, or a sign he was on Jack's world or the writers messing with us? no idea