Gate agents cannot ask "why" a passenger needs to preboard, but may only ask the following two questions:

Do you need assistance down the jetbridge?
Do you need a certain seat?

If the passenger answers yes to one or both, they may preboard. The reason they answer these questions may or may not be apparant and is really no one elses business.

Preboarders cannot sit in the OWWE. That is one reason why a FA is standing in the OWWE during boarding.