I need to catch up with this thread more regularly. I'm really sorry you are having such a horrible time of it lately and really pleased the lovely ukaps community have stepped up for you.
From what I have read and conversations I've had with others, tb is supposed (at least confirmed cases) to follow the path that your vet has outlined, ie a long time period of slow deterioration. The problem is fish can be so annoying and fit multiple diseases at the same time, so luck is always the most valuable thing with any treatment.
I've done skin scrapings on koi but not small fish. The trouble is identifying what you are seeing, unless you know an expert. It can be really really hard to 100% Id anything unfortunately so you still might be down to your "hit it with everything" solution. Popeye, the new symptom is just another sign that something bacterial is at least happening but it's probably a secondary infection.
I've not watched this video before but humble fish is a disease god in the marine world and if he thinks it's helpful it probably is. It's gonna be tricky with a small wriggly fish though, so recently deceased might be easier.
BIG THANKS to Dr. Charlie Gregory for providing this information. His website can be found here: https://www.aquaveterinarian.com There are some good wet mounts (and other disease info) here: https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fishes
www.ultimatereef.net
I'll try and stay more updated and keep my fingers crossed for you.