So after a 5 psi rise at your filter, you should backwash then add more DE. I agree that the pressure rise is an indication that you are filtering more junk. And if its noticeable as well, it shouldn't take long to clear.

It takes a few days for the CYA to dissolve enough to register on the test. You could start getting it in there now so that when you jumped through all the other hoops your test will be pretty good. With low CYA you will be losing more FC than you need to during the day. Remember FC tracks with CYA - the higher the CYA, the higher your FC needs to be. Check out the best guess chart for guidance.

Sand from concrete? I imagine you mean from your concrete deck. If you're getting sand from your filter in the pool, your filter will need a look. It should have returned some of the DE, but maybe its a backwash issue. I've read about a "rinse" cycle after backwash??? I'm not familiar with sand filters except what I've read here.

I can't help you with the sand filter or the SWCG. I'd suggest that you start a new thread for each of these if you need further help. There's a lot of pretty talented folks at your disposal. I'd post specific make and model number of the device along with current test numbers. They will ask you anyway.

Let me know how it goes.