Josh, welcome to the forum.
I've never done the plastic sheet thing but it would be the most efficient and safe way to dilute water. Trick is to get a big enough sheet to keep the top and bottom layers from mixing. You would drain from the bottom and fill the top layer and let the sheet sink during the process. You could virtually do a complete water change without risk of popping the pool. I don't know what might be the best way to stick smaller sheets together. Possibly layers of duct tape may hold long enough. As for the CYA testing, you could dilute a sample to get it in range of the test. ex: 1 part distilled or tap water (no CYA) to 1 part pool water, double the reading. 2:1 triple etc.
Al