No status on UAL and in an attempt to use otherwise worthless miles we are on an award tkt DTW-ORD tomorrow. Altho there are many seats available they are all Premium or some such, requiring payment. Since I do not wish to pay for this short hop the online UAL engine refuses to allow us to select seats. On the phone yesterday UAL said all seats would be available for selection at T-24 hrs but that didn't happen and today's UAL agent denies that.

So, if we don't have seats at the gate could they refuse to board us? Would that be IVDB?
I ask because I could buy an AA award tkt for roughly the same time and we have a connection to Hawaii 3 hours after scheduled arrival time in ORD.
Advice appreciated