Great thread and great job, bro. Read it all with attention.
If what I have been seeing with my fish is indeed columnaris, yours might not have been it because I see usually bad swelling and some mild redness here and there on a fish.
But there are 4 strains of columnaris and some act from the outside while others on the inside or so I read -
https://www.myaquariumclub.com/colu...out-the-types-causes-and-treatments-1689.html
My take is cautious on the diagnosis. It may or may be not HITH followed by a bacterial infection that may or may not be columnaris.
But the fact that the general antiparasitic treatment followed by the general antibacterial treatment worked appears solid.
It's hard to say whether the antiparasitic treatment was necessary but the antibacterial was absolutely necessary.
Thank you for the link and for this thread.