Thanks - Did that. They responded within the day with an electronic travel certificate on the condition that I leave them alone. There is no admission that what they did was wrong. "RELEASE OF LIABILITY
By acceptance of this travel certificate you release United; the operating carrier, and their respective employees, agents and representatives from any and all liability, claims or damages resulting or arising from the matters relating to your flight, compensation therefore or any related complaint."
Perfect. That's standard release language. Ignore it, you're not planning on suing them, so let them have the release, so long as the travel credit value seems fair to you.