M,
Sounds like you're doing very well. Proper test kit is important and you've got the right one. I get 18 bags required also. I'd put in about half and circulate 'till gone plus a couple hours then test again just to make sure you don't over shoot. You can use any water softener salt that has no additives. Morton make one that comes in pellets or granules both dissolve very quickly. It's also 99.7%+ pure. I read here often that may use the generic brands also with good success. No idea why they recommend the metal remover with no analysis. Is your water known to contain rust or other metals? Even with that I'd want to test first. I'll ask a few others more knowledgeable than me on pool opening to comment on that issue.
@ajw22 @JoyfulNoise is there any reason to add a metal remover when opening a salt pool?
You're asking all the right questions. Please do keep us posted!
Chris