I don't want to disrupt your train of thought as it is flowing and I am following with interest.
To finish off the OP theme, today I released both koi into a large natural lake on our property. Perhaps mother nature will heal them... or not. If they survive, and it's a huge if, I might get a chance to recapture them one particularly dry season, when all our ponds and lakes run almost dry.
Removing the infected koi is the best thing. Now I forgot what we're talking about.