You just hhappened to get a bad supv, and there is not much you could have done with him. You probably knew from the get go he wasn't going to do much? Can you ellaborate more on the process of how you got the extra charge down from $370 to $100? Do you know what he did to get you the $100?
The only other thing you might have been able to do was go to another concourse or area of ORD, and try to get a different supervisor. That is a long shot, but its all I can think of.

Do you know if the supv was a "service director" =first line supv, or a "supervisor airport operations" = a higher level supv/manager. It was probably an SD. You can always ask for an SAO. I think they might also call the SAO CS1 in UA jargon.

But you just got a bad one, and they are with all the airlines.

The only other thing I suggest to you is to try to get to know the agents at your home or regular destinations airport.

Its interesting HNLRR fixed things for you, typically the airport is more accommodating than phone res with any airline but maybe HNLRR is an exception.

Essentially you did the right thing = you kept asking until you got someone who interpretted the situation in a favorable way for you .

Also in general I've found RCC cannot do that much, unless in particular that RCC agent also works the gates/ticket counter, etc.