I don't know the ingredients in 'shock n swim' and I'm skeptical about adding things I don't know about. Too many voodoo mixes out there in the pool chemical world that sometimes have things in there that you don't want in your pool.

You should backwash whenever the filter pressure rises 8-10psi over your clean filter pressure. I don't think you need to vac to waste. Sounds like you only have a few spots and not a cloudy swampy pool. 'Vac to waste' wastes a lot of water.