A: Generally speaking, they don’t. Guests, once discarded, are not ever “returned to play” except under the following circumstances:
The guest visible on top of the discard pile is described as being “standing in the doorway” and is still available to serve, just like the guests laid out in the tableau in the center of the table. If someone serves that guest, thus revealing the guest below, that lower guest will now be “in the doorway” and available. If the guest on top of the discard is covered by the next discard, the previous top guest is covered, and the new top guest is the one that’s available.
The only way guests buried deep in the discard pile can return is if the guest draw pile runs out and the guest discard pile is reshuffled into a new draw pile. It can happen, but won’t necessarily happen in every game.