It is very rare that CO would cancel the original booking. We are instructed to add any new bookings (during IRROPS) onto the bottom of your PNR, just in case you make it in time for your original flight.

That being said, if you arrived at the UA gate before they consider you not going, you should have been allowed to board. It is not your fault that they were overbooked. Also they should not have sent you back to CO for assistance. At this point you are their customer even though UA doesn't like it.

Sorry you had such an ordeal.