Yes, They are different.
Example 1: leaves another player yet to act...Player 1 (checked)... if Player 5 who tossed out 12,000, were warned (preferably by the dealer) that it was a 55,000 wager that he had to call, I would consider allowing him an option to complete his bet, or surrender his 12,000. This of course would depend on no reaction from Player 1 (who checked, initially). If the action of the short wager of 12,000 were raised by Player 1, or Player 1 called 55,000...I would consider it too late for Player 5 because substantial action has occurred. As you know, some situations are similar, yet slightly different from another.
Honestly I might consider a different resolution for identical situations based on the history of the player, the silent 55,000 wager from the initial bettor, an unclear announcement from the dealer, or any other factor that might have led to Player 5's mistake. I call it a mistake because, I can hardly think of a reason for any player to deliberately put in a short bet. Why would anyone intentionally do that? He might be forced to complete his errant wager to some astronomical amount...or surrender his short call, just give it away!
Example 2: Action gets down to only two players. Player 5 should be allowed to surrender his 12,000 and not call the 55,000. Especially if Player 1 agrees to not force Player 5 to call. It's head to head and I've always been in favor of separate rules for head to head action.
One other thought on this TDA rule; Players use unofficial betting terms and gestures at their own risk. These may be interpreted to mean other than what the player intended. Also, if a declared bet can reasonably have multiple meanings, it will be ruled the lesser (RULED THE LESSER... YET FOR OR SITUATION...THROW THE BOOK AT "EM) value. Ex: NLHE 200-400 blinds, player declares “I bet five.” If it is unclear whether “five” means 500 or 5,000, the bet is 500. See Rules 2, 3 & 42. See Illustration Addendum.
The scenario above can raise other questions, too. Let's say that Player 1 does respond but only calls the short 12,000? Let's talk about this for a while...(Later)! These are my thoughts and I know they are not what the TDA wants.
So you understand that I know, in both of your examples Player 5 MUST call the 55,000 period! The reasons are: In Example 1 even though Player 1 checked, Player 5 is obligated to call the opening bet from Player 2. Likewise Example 1: Player 1 has made an opening bet, plus Player 5 MUST call because action was head-to-head.
I much prefer allowing a retraction of a short wager if a player was misled by another player, or misled by a dealer, as long as substantial action has not occurred.