Yes, Justine will be seeded. In fact, the grand slams are not ruled by WTA tour so they can seed whoever they want. Justine would receive favorable seeding just by being the past champion and an established top clay court player. In fact, most other top players would appreciate that, because then they don't have to face Justine in the early rounds.
They usually adhere quite closely with the rankings, so that wouldn't be a guarantee. Now that Justine's assured a seeding she probably won't be seeded any higher than her ranking at the time.
Yep, I think you are right for the FO and Wimby will probably follow suit. Of course it's very likely that both Kim and Justine will improve their rankings before then anyways and wind up with a decent seeding. I don't expect the GS to seed anyone differently than their current ranking.