I think you have the best suggestions here,
Sorry you have had to go through this, but at least if you get the fish out you'll have a definitive time frame (fallow period) to know you're safe going forward.
Having been in this exact same place, I strongly (STRONGLY) urge you to... slow down. You need a 4 month plan, not a 4 minute plan. I find that some survive Velvet and some don't, and I'd be shocked if interventions at this stage will change that outcome. I am assuming you want fish in the future, so your only plan is a four month plan. Get *all* the fish out of the tank, one way or another. Not to save them (although that may be a secondary benefit), but to save your tank for future fish and starve out the parasite. It is ok if it takes 2 weeks with a fish trap to catch them all (again, been there, done that, shirt is for sale on Ebay). Or take out the water until there's just a little left. Once the last fish is out, you let the tank go fish-fallow for the requisite time period (search other forums). Treat all fish in QT while you wait out the fallow period. This is a long haul approach, but you won't regret it. I am one year out from the protocol above, and there's not a spot on any of my fish. Not saying your LFS lied; but whatever they did, it clearly didn't work.
Sorry you have had to go through this, but at least if you get the fish out you'll have a definitive time frame (fallow period) to know you're safe going forward.