1. Yes, keep using those until you need more.
2. The amounts can vary widely with your water. If in doubt, try messing with 5 gallons of pool water and measuring the effect, then scale up to your pool size. Too little is always better than too much!
3. Draining water and replacing it is the way to lower CYA. There is a chemical way, but I don't think it is cost effective.
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