The gate for a routine domestic flight (staffed by 1-2 agents) is not the place for an extended transaction involving ticket sales. It is neither commercially feasible nor a good use of resources to do all of this at what one presumes is the last minute. If not the last minute, head off to a customer service desk and deal with it there.
The one issue which was remediable was moving OP's seat once he cleared standby and was assigned a seat. That is a "few keystrokes." Cancelling and rebooking e-tickets is not quite that simple.