The psi went back up to 15 in about 12-15 hours. Truthfully I'm not sure it's actually doing anything. It normally takes several days for my psi to get that high. When I backwashed the 1/4 cup out I didn't even see any green in the sight glass just white. Normally I would see lots of green getting backwashed out. Not sure if I should do it again. Didn't see any real difference in the pool clarity. It's been about the same for the past 3 days.
This is actually confirmation the DE is working. Right now you have a lot of junk to filter out. If it normally takes 3 days to reach 15 and it got it to 15 in 3 hours, I'd say it's working. You're not going to notice change overnight because your water is still pretty cloudy and you probably still have organics you're killing. If you think of the cloudiness in your pool as "stuff that makes my filter pressure rise", then you can simplify the equation by thinking in terms of "I might have to backwash 10 times in this SLAM". DE just helps you reach that faster. Now that said, it doesn't affect the chemistry problem you're also trying to solve - stuff will still try to grow and the FC elevation is critical to keep that going. Which will add dead algae, which will increase number of backwashes. But the quicker you get some of that stuff out of the pool, the easier it is to visually see (and thus brush/vacuum), the easier it is for the chlorine to do it's work, etc. I'd keep using DE, even though it adds a tiny bit of work.
The white instead of green is possibly explainable - right now you have both dead and live algae in your pool. Over 3 days, the sand filter collects a lot of both since the whole pool gets circulated many times. It's easy to see how your backwash would be green tinted. With DE, you backwash after 3 hours. This is not a lot of time to collect the live stuff since you kind of have to luck out, but the dead stuff that clouds your pool the most is readily available to be captured right away. But I might be mis-thinking this. At any rate, faster PSI rise is at least a definite sign it's filtering stuff out, and right now any tint to the sight glass (green or white) is a good thing.
Here's the pool today. I left the vacuum in so you can see how clear it's getting on the shallow end. It's been ungodly hot here so I've been vacuuming at night... then I realized during the day you can clearly see where I have vacuumed and where I haven't so I'm doing it during the day now too. Do I start taking overnight FC and CC numbers now or wait until I can see the deep end? Does it need to be crystal clear to stop SLAM?
Many people stop before it's crystal clear only because they don't know what clear is (but discover after a few weeks of TFP how clear it really gets). But if you can clearly see the bottom and detail of the main drain, CC is near zero, and FC holds overnight, then you at least don't have anything growing anymore and can lower back down to normal.
Given the number of problems you've had to deal with, I'd encourage you to stay the course right now. Even though it's been awhile, it's not really started truly working until just a bit ago when your filtration started working effectively. If it's still green and cloudy in another week, then some SLAMs have suggested letting stuff settle to the bottom to vacuum more effectively, but I don't know that yours is to that point yet. I'd say to use the DE as long as you're around to monitor it, and try to get that stuff filtered out.