If I were you, I would stick with 12C as it is free. As you are already in the EC zone if there are any seats free you can always move there afterwards.
I was under the impression that all seats (including emergency exit seats in the EC zone) were only accessible for those eligible (full-fare ticket/pay/FB Plat) for EC.
The price for an EC seat on AMS-LAX is €120, so as FB GE you would pay €60.
Gajan