If I and my opponent both have a Torn Hero in play, and I commit his Torn Hero with a Stun 1-enhance, and he chooses to commit two of my foundations that are not Torn Hero, can I then use the R-ability and:

1. Pay 1 momentum, and commit Torn Hero instead of any other cards otherwise commited.
3. Pay 1 momentum, and commit Torn Hero instead of one other card otherwise commited.
3. Can't use the ability since it says "a card" and Torn Hero commits 2 cards.

Also, how would it work if I got stunned by a Stun 2 (or more)? Can I choose Torn Hero as one of them and then use the R-ability to commit it instead of the other one instead?

2. (that you comically mistyped as 3!)

You R: with Torn Hero to replace the commital of ONE of the two cards. The other still gets committed as normal. You would need 2 Torn Hero to protect 2 cards from getting committed. Note that the commit 2 would be declared as one, so he couldn't try to commit one, then after you redirect to Torn Hero, try to commit it again.

Thanks for the answer. And the typo is because I first made it a list with 4 choices, but I remembered that it couldn't be that choice (only one response per trigger per card).