You are 100% correct, so to be fair to all and treat everyone equally, do away with the priority board first process, and everyone gets in line first come/first serve. That way everyone is being treated equally. But IMHO that doesn't make much sense, though it would be equal treatment.
I'm not trying to kick the hornets nest, that's already been done in previous threads. But, I think what the OP was trying to say, and makes some sense is that those needing accommodations can pre-board with one other person, the rest of the party waits in line with the rest of the crowd. If they get on the bus that's wonderful, if not then they would need to wait. The person needing the accommodation would be assisted, no discrimination in my eyes.