Nick:
There are two required parts to TDA Rule 9; a) a player MUST be all-in, and b) ALL betting action for the hand has to be completed.
If there are only 2 players in the hand and one of them has declared "all-in", part 'a' is satisfied. Since there cannot be anymore betting action, part 'b' is also satisfied and both hand should IMMEDIATELY be turned face up. If player 'a' tries to muck his/her cards, the dealer should catch those cards and turn them face up until the hand is complete. Remember "Cards Speak".
If there are MORE than 2 players in the hand and one of them has declared "all-in", then part 'a' is satisfied. If none of the remaining players has more chips than the 'all-in' player, then part 'b' is again satisfied as there cannot be any more betting action and again all hands should IMMEDIATELY be turned face up. If any player attempts to muck his/her cards, the dealer should catch those cards and turn them face up.
If there are more than 3 players in the hand, one of whom has declared "all-in", then part 'a' is satisfied. If at least two of the remaining players have more chips than the 'all-in' player, then part 'b' is NOT SATISFIED. NO hands should be turned face up as there is still betting action pending for a side pot, etc., etc., etc.
In the example above, ONLY when all betting action has been completed should the dealer instruct those players involved in the side pot to turn their cards face up. Player 'a' should be told to hold action and leave his/her cards 'face down', until all side pot decisions have been made and side pots awarded. Then and only then should player 'a' reveal his/her cards, they cannot be mucked and if this player tries to do so, the dealer should prevent those cards from going into the muck.
I believe that it is contrary to the intent of the TDA rule 9 to allow player 'a' to muck his/her cards.
I totally agree with the rest of your post dealing with asking to see a called hand and limiting that right.
Hope this helps!