Yeppers, sounds correct.

Ouch. Yep, that's why if the opti comes off, the seals get done. Doing one or the other just means doing it again.

As for the other o-ring, it could be a spare. I don't see any other spot it could...

Yep, that's correct. Just make sure you install all those seals correctly (the one on the water pump is especially critical to get correct and there are plenty of ways to do it wrong), and that when...