Advice for a Specialist in their Secondary Zone who Is at a crossroad in their Army career regarding promotion board?

Everything you’ve said is wrong, just very wrong. You have been grossly misinformed about everything. If you don’t plan to reenlist, no big deal just ETS. Everyone gets out eventually, start the STLTAP process now. If you were to reclass you would have to reenlist anyway. All reclasses are conducted in a TDY enroute status, so if you reclass you will be adding time to your contract. I think you have misunderstood the reclass rules for OCONUS as they were explained to you. First, you cannot reclass to 42A because they are over strength. Second, your career counselor must find a school date that matches up closely to your DEROS date. It’s almost impossible to find a job you want, a job you qualify for, a job that has class seats, and those class seats are within a small window to your DEROS. In three years OCONUS I never reclassed anyone who wasn’t an application MOS like SF of CI. You’re better off waiting till you get to your new unit and Reclassing once you’ve been on station a year. But as you’ve said, you can’t wait that long, you have bigger plans and within two years you will be on to bigger and better things in the civilian sector so I understand the Army can’t offer you any to compete with that.
As for the Bar for the board, I suggest you read the promotion regulation and the Army Directive entitled retaining a quality NCO Corps. You are not even eligible to attend the board without a waiver. If your command is pushing you to attend the board in your secondary zone it means they believe in you enough to waive the minimum requirements for you to attend. The bar only applies to people approaching MLI which isn’t for almost two years for you.

Stop painting yourself as a victim, you have lots of choices. Just because they aren’t the choices you want doesn’t mean you are without choices. If you want to reclass it means added time in the Army. Whether you reclass now or later, it’s added time. If not, then ETS. No one is forcing you to do anything other than complete the terms of your initial contract.

Go to the board, promote, and ride out your last 1.5 years, even if you aren't in love with what you're doing, then ETS with an RE1. The military is full of people who were never going to reenlist or were getting out for good. Give yourself that option in case life doesn't go the way you planned it. If nothing else, it will give you peace of mind to know that you always have it to fall back on. Rallypoint is full of people asking how they can re-enlist with an RE-3.

Have you thought about going to the Reserves or Guard? You may be able to switch MOSs that way.