As long as your chlorine never drops below your minimum per the CYA/Chlorine chart, you really don't ever have to shock the pool. Honestly, I seldom do. Of course the pool stores and pool chemical bags tell you to do it weekly to increase sales!

I have never had an issue with grass being killed from backwashing into my yard.

Regarding the stuff that keeps reappearing along the seam lines, it may very well just be dirt. I see the same thing in my pool. I vacuum very slowly and it appears to all be gone but then when the water quiets back down, it is inevitable that there will be a little dirt that had gotten into suspension that will resettle to the floor around the seams. Its no big deal. Much as we want to, it is impossible to keep an outdoor pool totally dirt-free.