It sounds to me as though UA changed OP to the later flight, rather than protecting him on it, while preserving the original reservation. This is also consistent with its standard practice, e.g. not protecting and preserving for other than GS and other very specific situations.
I would send a short note to UA. Cut out all the irrelevant stuff and note that UA should not have offloaded until T-15. UA may toss something OP's way, but that's really it.
Definitely save BP's to one's wallet as they can't be pulled back by the carrier, although you do have to go in and manually delete them at some point as they tend to pile up.