Could Arcade mastermind the damage Fabian Cortez would give him for using the perplex? The way mastermind reads I would think so...but I want to be sure.

No; its says anytime THIS CHARACTER IS DEALT DAMAGE.

I think the damage can be masterminded unless its the unavoidable option.
because its damage dealt.

That's the way I was reading it...

Quoting is fun.

If you roll a 4 or 5 with Power Amplification while targeting Arcade then Arcade can Mastermind the damage from himself to an adjacent character because it is normal damage.
If you roll a 6 with Power Amplification while targeting Arcade then Arcade cannot Mastermind the damage because it is unavoidable damage.