You can drain that way, IF you make sure (a) the ground water level is BELOW the level you drain down to. (Not a problem in most of the country, right now!) and (b) that there's at least 8" of water above the HIGHEST horizontal liner covered point. (So, if you have vinyl covered STEPS, you can't drain at all!)

Chemically, you usually have to use a mixture of both stabilized and unstablized chlorine, in order to avoid stabilizer level creep.

Get a K2006, learn to use it, tell me what your test results are . . . and then I'll explain using cal hypo without CALCIUM creep. Till then, use PLAIN 6% household bleach.