As others have said...
In the NFL, no, the defending team cannot score off of an extra point.
To answer your other question, the rule differs between NCAA football and the NFL. In college, the defending team CAN advance the ball and score a touchdown off of a botched extra point (or two-point conversion) attempt.
-YendorMike
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
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