All-in mucks, need to show?

[pfapfap]

Playing limit cash game, three-handed, one all-in on the flop. All-in mucks before river action. Live opponent mucks to my river raise. Dealer says I have to show my hand to get the pot. I laugh and say I have the only live hand. She says, "I know, but he was all-in." I felt having a conversation about this with a dealer in the box would be counter-productive, so I tabled my flopped top two (SHIP IT HOLLA!) and moved on (tho' I'd rather have left people wondering), but this brought up a couple of questions...

(1) Do I need to show to get the pot in this situation?

(2) Since the all-in player mucked before live action was complete, and the remaining player folded to a raise, is IWTSTH an option?