I regularly acquire fish from Petco and they almost always have velvet (especially nowadays). After a lot of trial & error I finally developed this protocol for treating them that has a high success rate: https://www.reef2reef.com/threads/velvet-amyloodinium-ocellatum.217570/#post-2499437
An addition for your writeup: Acriflavine is also a mutagenic carcinogen and should be handled with care like Formalin ie gloves, possibly mask.
Sarah