10 lbs of trichlor should add about 55ppm CYA to your 12,000 gallon pool. 50 or so is fine. You may want to run it higher if FC won't stay between Min and Max on the BestGuess chart with daily addition of bleach.

As for the filter, bugs should not go through a sand filter. Something's wrong(I'm pretty sure you know this). It seems to me you would have noticed if 200lbs of sand had been backwashed out. Perhaps the multivalve isn't seating properly or the spider gasket has gone bad. Can you convieniently check the filter?