Update [9pm local]: Never mind. She didn’t survive the panic transfer to the hospital tank. Stopped breathing about ten minutes later. Surely it was just the final shock after days of sickness and no eating and a very weakened state.
All the information is very helpful, though, thanks. If I ever have a problem like that again, I can jump on it much quicker. Freshwater dip within a day of noticing this kind of abnormal behavior. And I should probably stock some of the medicine you mention, so I have it in house if I ever need it again. And lastly, I now have confidence to forcefeed a seahorse if it stops eating. I never would have tried that on my own. Almost sounds like fun (if under difficult circumstances)!
Ah well. Thanks for the multiple replies. My long delay before even starting treatment probably doomed this particular specimen from the start.