Question 1. Body Thief. I assume Intelligence contest is an ability check of Int vs Int. Correct?
Question 2. Brain dead. I assume if the pc becomes brain food, and in Tier 1 you is out of luck to raise your pc.
To be clear, greater restoration does not need to be changed to reverse an intellect devourer having reduced a character's intelligence score to 0 - ending any reduction to one of a character's ability scores is a standard effect of the spell....changing it to be reversible by Lesser/Greater Restoration would be fine.
Intellect Devourers are easily the worst designed monsters in 5e, with multiple save-or-die abilities that don't allow Raise Dead on a supposed CR 2. My advice: replace them with more appropriate monsters that aren't quite so ridiculous.