Can Tress Exhaust Aryx Variation during an attack to gain 2mp? and if so why?

No. "During your activation" is a timing that's only possible when you're not resolving an action or another "during your activation" ability.

(Besides, she would not be able to spend those movement points anyway, because they would not be gained as-part-of-a-special-action and thus would go into the movement point pool, and spending movement points to move is another "during your activation ability".)

Yeah she was using the MP after the attack but gaining them during the attack. So either Way it's a no