With CYA/stabilizer including the CYA in the chlorinating tablets can take up to a week to register on a CYA test. It may have still be from using tablets ??
CYA is always a difficult test for me and it also might be what level you were holding the sample and how bright the sun was. Have you seen the video - >
https://www.youtube.com/watch?v=NxaqoW-_MCs&list=UUNFkPFCkt9Y_YYNAeLtUr1g
You really don't need to change sand, sorry. It is a very good idea to deep clean the sand but the sand doesn't wear out. How to - >
http://www.troublefreepool.com/threads/73192-Deep-Cleaning-a-Sand-Filter
The folks who sell the really nice TF-100 test kits also sell gauges next time you're in need. It just screws out and you put some teflon tape or a dab of pipe dope on the threads and screw the new one in.
http://tftestkits.net/Pressure-Gauge-Glycerin-filled-2-5-Bottom-Mount-p77.html
How to - >
http://www.youtube.com/watch?v=RhqNbs5xYJc
If your clean pressure, the pressure of the filter after a through backwash is 12psi then you should let it go till it hits 20-25% more (15psi) before backwashing. If you backwash too often, before you hit your pressure the sand filter can actually be
less efficient. Letting it collect some debris and pressure it's actually plugging up a bit and catching smaller particles.
As for your SLAM starting over, not really. There is a certain amount of waste in letting the FC drop. If it drops some then the chlorine isn't strong enough to kill, only keep the algae in check. If it drops enough than it does allow the algae to start to reproduce again. The more consistent you are at holding the FC up at the SLAM level the faster your SLAM will be and the less overall bleach $$$ you'll use doing it. That is why everyone stresses the "M" in SLAM as it's MAINTAINING that FC level to kill algae.
One thing you can try on the weekends or when you are around to monitor pressure is adding DE powder to your filter to make it more efficient and catch smaller particles. The trade off is the filter can get plugged up quickly so you need to keep an eye on it to see how yours goes.
http://www.troublefreepool.com/content/133-add-de-to-a-sand-filter
How about a picture of the water ?? A photo taken at the middle of the steps looking down helps us figure out the clarity and distance using the steps as a reference.