1) Yes, rejuvenation potions can be used between battles of a dungeon to revive dead mercs, not just heal your own fighter. I like to give the merc a boost potion before giving the rejuv potion, because they will automatically fill up to the boosted levels. There is no way to leave a multilevel dungeon part-way (to heal in town) and return to the same spot in the dungeon. If someone healed between battles, they did it from the dungeon, not the town.
2)What you are describing is impossible. It you are sure that it is a multi-level dungeon, than only the mercs that were uses in the each level of the dungeon can be used later on - they cannot whip out mercs at the end that weren't used at the beginning.
Hope that helps!