Mating O. Briareus - Video and Summary
Unfortunately, it is choppy, grainy and poorly focused but it tells the event.
Conclusion:
Was the mating a success?
I have some concerns. Watching the O. mercatoris mating I could see a "shoulder" being formed on the male where the semaphores were gathered before being sent down the hectocotylus, I did not see any swelling on Tatanka's arm.
After they separated and rested, Tank slowly made his way over to Cassy. We decided to end the meeting at this point, again with concerns about agression but he seemed only to be reaching for her with an exploring arm and not in the attack mode we saw with the introduction. In retrospect, we probably should have allowed them to remain together longer.
Should we attempt a second meeting?
I hope not but I will try to get Roy's input on the chances of this being successful. They stayed quite still for close to 15 minutes and there was a definite end to the coupling (ended by Cassy) so I am hoping this means they were mating but in addition to not seeing a swelling in Tank's arm, I could not positively see insertion of the arm tip. (The video did show the tip waving around and seaching).
Additional note
I have never encountered issues with brissel worms leaving their bristles in my hands even accidentally handling them during water changes. I don't wear gloves in tanks without stinging corals just because they make working in the aquarium difficult, however, I will never clean out sand substrate again with my bare hand. I felt the bristles soon after cleaning the tank and now my hand is itching like crazy. I expect it may swell by morning.