First I am an strong supporter of UA and have had several very positive flights on GO JET. I think they do a good job. UA certainly does have the ability to assign both via the internet and at ezcheck in an exit row for GO Jet. I think it reserves exit rows for 1P and above passengers which may be why you had a problem. I have done this many, many times.

Regarding an agent at the gate, I really think that you are naive in thinking a call center whether in India or Indiana is going to have any idea of how to get an agent to your gate, or assign a seat for a plane that is clearly now under airport control being less than 30 minutes from scheduled departure. A better option would have been to use the airport paging phone and page a united agent to the paging phone. By the way both your calls to call center gave you the correct answer, for a non elite status traveler this needs to be done at the airport. You may not like the answer but that is the answer.

I agree UA should have had an agent at the gate at least 30 minutes before departure, but complaining about UA about the condition of the ST.Louis airport, the TSA facilities, and the over priced airport food is just not logical.

UA doesn't always do everything right, but I don't think they are anywhere near as bad as you state. UA may not be perfect but my experience with over a million miles on UA and 2 million on AA says that they are as good if not better than most of their competitors for frequent flyers in Chicago.