Can you use Feint between enemy attack reveals shadow card and the attack resolution?

Nope. Feint prevents the enemy from attacking, so it needs to be played before the attack. If you already revealed the shadow card it is to late.

There is a window to play cards after the shadow is revealed, so you could heal damage on the character before they take more or sneak attack Gandalf in to deal damage to the enemy before the resolution of of the attack. Just does not work with Feint.

And in case you're interested, I think the timing of Feint is addressed directly somewhere in the FAQ!