OP here,
Just following up to say that five weeks after I mailed in a complaint letter to US Customer Service, I finally received a response back in the mailbox today.
My letter to them specifically outlined the procedural errors that happened, the rules that govern those procedures, and what the correct outcome should have been.
The response was clearly a form letter, dated three weeks ago, which feebly explained how some flights are oversold based on expected/historical load. Whatever.

But the good news is they included a round-trip travel voucher good for one year in the lower 48. Very pleased about that. But they spelled my name wrong on the voucher... ahhh, US.
Thanks to everyone here who helped out and steered me in the right direction.

AC