Fairly certain Symbo version of Cleanse, remains if they enter an instance. So you "could" have someone log on a paladin outside of ToT, and use them as a symbo target and have the druid never release to lose the buff?
Use army on the 3rd door, i think this should result in less diseases on your raid.
On the 4th door the shamans only cast hex (curse) once they're dismounted, thus you can control when the curses are going out. Personally we burn down the 1st shaman and then keep the 2nd and 3rd until the door has been shut. We decuree tanks 1st, then dps, then healers.