So as I said, Plus, there are 5 Sacked! cards. Is this the extra card?

Mine only has 60 cards (including the 5 Sacked cards).

Are you sure about your count? Maybe you just got lucky! (Especially if the extra card is a player card)

....Or is that extra card the "missing" 3rd Quest Card that people are asking about?

(since quest card 2B doesn't specifically say anything about the players winning when they complete this stage, some are curious if there is in fact a 3rd Quest Card that's supposed to be there, or if FFG simply neglected to put the 'winning' clause on card 2B)

As far as I know x5 Sacked is correct, you can go to www.lotrlcg.com and set the Filter to "Shadows of Mirkwood", check the box "List results" you'll get a list of all cards from Carrock including how many copies of each of them is included in the expansion. This should reveal what card you have surplus.

There's no missing quest 3A/B. The adventure pack contains 60 cards. The adventure structure makes sense with the current 2 quest cards. What would a 3rd quest add?

It's only wild speculation that says there should be a 3rd Quest Card -- and that's simply because every quest up to this point has specifically stated that players have "won the game" upon completion of the final quest card. And the phrase is notably absent on Quest 2B of Carrock.

And since the Original Poster claims he got 61 cards in his pack, I'm equally curious as to what it might be.

Unfortunately, that was his one and only post. Hopefully they'll come back and tell us what that magical 61st card is... Or tell us the they simply miss counted