Just my opinion. Your doing tooooo much. It is VERY common for tangs to get ick. Especially when transported into a new environment. It is 99.9% stress related. Every water change, treatment and food change is causing more and more stress. SLOW down. It's the fish that will shed the ick as stress is removed. Give mild flow, nothing direct feed nori on a clip. Stop looking at it every 2 minuted like a monkey in a cage. A monkey would get so stressed it would probably throw poo at you! It,her, not the ick needs to relax. I
brought home 2 blue tangs in perfect health from a very well established tank. Did all the usual good stuff. But in main tank and I don't QT. They where covered from fin to fin (T say head to tow) in a day I mead really heavily covered. I expected it. They where eating with no other problems. Like you said your lady was. I didn't do anything, no treatment no water changes, and no lookin at them on a constant basis It took them 2 plus weeks to get over it, all on there own. After another week of absolutely no spots i did a small water change, more for the corals than the fish. Non of the other fish got ick. and there has never been another one since. I have at-least 3 more tangs to get, blond, powder blue and, well I'm still thinking. If this is scaring the hell out of you, don't get a powder blue. I expect it will get ick as this is , again very common.
I will say in the beginning, looooong time ago I freeked out with the tangs getting ick when they came home. I also killed them with too much love.
Good luck.