You are wrong IMO.
I disagree with you on both points.
Even in the screen shot you showed, Mitchell might as well tell Dobbins what he wants for Christmas because he is sitting in his lap.
As a DT, more specifically a NT, you cannot ever allow yourself to be driven back into the LB's -- double team or not. There are ways to combat that.
On your second point, I disagree again. There was misdirection action. He was not shooting gaps or blitzing. He was reading the play.
He would have been able to come back across a lot more cleanly had mitchell not been humping his leg on that play.
And I know you never said, "come back"....and that is why you are wrong. Go back and look at the play and you can see the RB take a misdirection step to his left...Dobbins is keying the RB wo when he initially went that way so did Dobbins...When the RB came back right to take the handoff Dobbins proceeded to try to come back, but by that time Mitchell was kissing him goodnight...
Demeco was keying the FB and that is why he flowed straight towards the gap. The FB took no misdirection step. Demeco did what he was supposed to do which was attack the FB's outside shoulder and try to close the gap down.
Go to the sequence of snapshots, the RB did not start left; he went right and followed the FB until he saw nothing there and cut back around the FB's block.
Here's the sequence:
http://s1217.photobucket.com/albums/dd384/76Texan/PS%201%20Jets/SG%2019yd%20run/