@leena,

Looking through all you wrote it would seem to me that you've done quite a lot of work on your sand filter to no avail. Here are some of my thoughts/responses -

1. DE Blowing through filter - I would hazard a guess that the DE did not work because your old filter media was probably suffering from deep channeling. That's a condition that only occurs in sand filters whereby old media that isn't cleaned thoroughly develops deep channels throughout the volume of the sand. Think of it like a cracked mud bed that you would see in a dry lake. The DE that you added to the skimmer simply shot right through the deep channels and straight back into your pool. Sand filters should be deep cleaned (that's not the same as backwashing) at least once per year or more frequently if you have heavy debris loads. Deep cleaning is a labor intensive process whereby you stick a garden hose into the bottom of the sand filter and to wash all the "gunk" out of the sand and, as needed, use a long wooden pole to break up chunks of clustered sand being careful not to damage the laterals.

2. Oversized pump - without more detailed knowledge of the make and model of the sand filter, it's hard to know if your pump is properly sized. However, if you think it's oversized, I believe you on that. And it is definitely not good to have an oversized pump and an undersized sand filter or vice versa for that matter.

3. Spider gasket - I don't think your problems are with a spider gasket as that would manifest itself as weird leaks between different settings on the multiport valve, eg, water leaking from the backwash port when valve is set to filter mode, etc. However, it is always a good idea to check the spider gasket when doing filter maintenance to make sure there's no cracks or tears in it. If you take the top of the multiport valve apart, you'll see exactly why it's called a spider gasket (looks kind of like a spider's web).

4. Pool leaks - I'm not an AG pool owner so I don't have much to say for finding leaks. I just hope you can locate it and fix it easily. Makes me wonder if something like red food dye would work as a tracer to see where a leak is? My guess is on a tiny leak it would be hard to do. Since it's an AG pool, can you see tell-tale water stains around the pool that might indicate the location of the leak?

Good luck and keep posting, I'm sure all of us here can help.