A complaint to UA? It's worth a compliment to UA for the FA correctly enforcing the rules!
Weight and balance issues are not the passenger's concern. The pilots and airline operations are well aware of the weight and balance requirements of the aircraft being used and if there is an issue then they would move people around.
On UA mainline flights you can generally pay for an on board upgrade with a credit card but it's not available on Express flights like the one you were on. You have to pay at the gate or online advance of your flight if you want E+.