I don't see how it's a violation of the CoC to:
(1) buy a discounted round-trip ticket
(2) after arriving at your destination, decide to change your travel plans
(3) attempt to change your ticket, but find that it's not changeable
(4) purchase a one-way business class ticket home on the same carrier
Maybe OP owes UA the fare differential between what they bought (r/t) and what they flew (o/w). Maybe. But I don't see how OP violated the CoC unless they are lying here and always intended to throw away the return. I have no reason to believe OP is lying, since changing one's travel plans is something that happens frequently.
You cannot hold that buying a no-changes ticket legally requires you to board that flight. If it's "use it or lose it", there has to be a (legal) way to "lose it".
I agree 100% the OP has little to gain from contacting UA and should probably just no-show for the flight (or hope it gets cancelled/delayed). But I don't understand all of this "you're violating the CoC by changing your plans!" nonsense.