Sorry Had to look. F'n gnats Oh My Science! I thought you were kidding. What a headache you have been dealing with, F'n frustrating too. I had a bout of the gnats and was victorious. DE from what I read was not good because it kills all organisms that are good as well. I tried the Safer spray crap too.
I later found that I really had two problems. I had the gnats which as you know eat the roots and seriously inhibit growth of the plant. But I also had THRIPS which eat the sap out of the leaf and inhibit stored energy from being delivered to the plant. I thought I was seeing gnat larvae on my leaves but, NO it was thrips. I spent months fighting gnats when I should have been fighting thrips.
I lot of people mistake thrips for the gnat larvae. Thrips will be yellow-green and you will see black poop around what looks like weird white-ish scratches on the leaf under a magnifier. Those scratches turned out to be where the thrips suck out the sap or chlorophyll of the leaf. Almost like leaf miners. If you see a yellow-green thing just sitting there you might think its dust. Try to touch it and see if it starts to run away they can also hop like a flea they do have wings (which is why I think people get them mixed up). If it does then you have thrips. There is one product after hundreds of hours I found that is OMRI certified for organic. Look for Monterey brand Garden Insect Spray concentrate with "Spinosad" Make sure it says it has Spinosad on the front label with a cartoon worm on a tomato. I got the small pint concentrate uses like 1 oz mix in 32 ozs of water - so happens to be the size of a good water spray bottle. I sprayed the under sides of the leaf first then the top of the affected plants, until they were drenched. Do it again 4 days later due to the egg cycle,...then I did it one more time another 4 days later. My plants got immediately better in a day after the first application and in few days they looked like nothing was ever wrong I did that last spray out of spite and frustration. Anyway, never again have I had a thrips. I got 13 oz of concentrate still.
Gnats: I solved that with Mosquito Bits sprinkled on the top of the soil and in the trays, each time you water it will activate Bacsillus Thurengis or something like that which kills the gnat larvae while not harming other things. I also tried lady bugs - don't do that you will just feel sad about 1500 lady bugs running in circles and dying everywhere. I don't see gnats anymore either. Oh yeah I also tried Hydrogen Peroxide but that also kills all the micro-organisms which help the plant get the nutrients it needs in organic soil. So then I had a nutrient deficiency. And covering soil with the sand thing - nope doesn't work.
I'm in Colorado which is dry so you may be dealing with something I am not familiar with due to the humidity. Also I skimmed through and saw some pics of plastic covering the pots while leaving the stem and leaves out. I don't know how oxygen gets to the roots in that scenario but like I said I didn't read it.
If you solved the gnat thing already then at least some others might see something they hadn't thought of.
Good Luck!