I will start with the shop. I hope they do see this thread, even IF they have a different story,
The point is that it came across the way that the OP stated, which is the real problem. So it doesn't matter what the instructor actually said or did, for an instructor to be so ignorant/arrogant as to even imply that he MUST BUY different fins to procede with his class has absolutely no place in the world of consumer relations and he needs to find a new job. Yes, I know that sounds harsh, but this is exactly the type of high pressure sales that almost all dive companies take issue with, not to mention is bad for repeat business. If this were me, I would go to the LDS owners and 1) make sure the owner understands the sales pressure that was applied and how it negatively affected my view of the operation, 2) demand that the owner makes it right by assigning a different and more qualified instructor for the remainder of my course
As for snorkelling, the Originals, Foil Force, and Beaver Force, and Rip Force, all work quite well for snorkelling. Personnally, I have found the Foils work the best for me. I often pick the Extra Force TanDelta though... but I normally am freediving more than I am snorkelling.