Arien

What is the timing between this Ultimate and the "Before the attack" on Arien's Gold?

“Before the attack” is part of the attack action. So that happens second. Somewhat unfortunate naming overlap.

On Arien's card, does "directly behind" mean in the direction that the attack is coming from?

Imagine drawing a line of hexes from Arien towards the target, continuing in that direction. "Directly behind" are all spaces of that line, except the one with Arien and the one with the target.