Well, the mandarin is still alive. Still breathing heavy, and he's lost some of his slime coat (mandarins always have thick, heavy slime coats, making them resitant to parasites, normally), and no one else is struggling. The mandarin is in my 10 for now--he's not eating, but there's about 2 days worth of copopods if he decides to try, and he will take prepared foods, so should be okay for now.
The reef was torn down last night to remove all the fish, and they are all (including the trigger) currently in a 50, being medicated for parasites. Lacking any other symptoms, despite no known vector, this seemed the most likely possible cause. So far they are all active and behaving normally--the damsel got nipped by the trigger, but that's the only conflict thus far. The main tank inverts and corals are still thriving--xenia pulsing, colts out and happy, etc.
It's been a rough couple days--didn't get to bed until after midnight last night, since the meds came from Ft Collins. Spent most of today tearing apart the water storage system to clean it out, just in case, and trying to normal weekend household chores as well.
On the plus side, tearing the reef apart uncovered a very large red brittle star never seen before. Must have been a hitch hiker many moons ago that thrived. Also found some nifty sponges, and several smaller brittles.
Thanks for the support, all!