I've been following this thread from inception and have a couple of thoughts.
First, I'm sorry to hear about all your troubles lately.
Second, I hope it is not Ich still in your tank that you are seeing on the tang.
Third, from everything that I have been able to research/read about Marine Ich, the extreme minimum amount of time to cure Ich with hypo and be relatively sure is a little over 8 weeks. I don't think you kept your tank at a hyposalinity state long enough.
Please don't take this the wrong way as I am in no way an expert at any of this. I am sincerely hoping to shed some light on this problem to help solve this problem. As I understand the process (in a nutshell)...
Lower salinity to 1.008 to 1.009 sp. gr. within 48 hours and maintain.
Closely monitor pH as this easily changes in a hypo environment.
Maintain hypo for 4 weeks after the last spot is seen.
If after 4 weeks with no spots, remain in hypo for 4 more weeks to be mostly sure the Ich is cured. There is no way to be 100% sure.
Start raising the salinity back to normal. Raising sp. gr. is more stressful on the fish than lowering it. Raise it no more than .003 sp. gr. per day. This should take at least 6 days to return to normal salinity.
Hope this helps if you have to go hypo again. Hopefully what you are seeing is not Ich.