No, there is no guarantee you will have an opportunity to book with a 10% discount once you leave the ship. Randomly, some people get a post card offer, but not all.
If you want mid-ship, just decide how high you wish to be. Basically, the difference in categories is different decks; and "A" on a verandah category is mid-ship. Lower category numbers will be higher decks, higher category numbers are lower decks -- again with the "A" designation being mid-ship. Category numbers will shift to other decks with differing letters, generally referring to forward or aft locations. Again, if you want midship, just decide which deck(s) are of interest and ask for it that way. If you want an oceanview at midship, you will have to be on deck 1 or 2.
I think part of the "popularity" of 7As is that there aren't many of them so they book quickly, plus it's the lowest category of balcony rooms.
Enjoy your cruise!