That last one has been the biggest problem, and forced me to (eventually) empty the filter of sand and re-level the base in order to get it to line up with the backwash valve (which is actually called a "Vari-Flow Assembly"). The problem was that the coupler had cracked (see pic below) and started to separate. Ideally I would replace the coupler, however it seems that it is not possible to replace the coupler without replacing the valve (or finding the correct offset parts for the valve), since the coupler is a captive nut. The reason the coupler cracked in the first place was due to the filter slab and filter being out of level (so the filter was pulling away from the valve).
To "fix" the coupler I glued it back together using superglue, and time will tell if it will crack again or not (but it was clamped for 2 days to cure and the crack was clean so it was a best case for superglue to work). Due to the issue with the slab though, it would surely crack again if I forced it together, so I had to level the slab. My dad, remembering back to when he helped out with the Great Pyramid of Giza, suggested using a lever (specifically a 2x4 or 2x6), which was a decent idea, but due to the close quarters, not workable. So I recalled seeing some 300 lb air wedges at Harbor Freight, and thought they could work, but I wanted to get some with the largest capacity possible. I bought these air wegdes (which have a 500 lb capacity), and they did the job.
My method was to clear some space for the air wedges to slip under the slab (or pavers under the slab in this case) and then pump them up evenly. They become very tough to pump when they start getting to the limits, and I certainly pushed them since, although the filter was drained of water, it was still full of sand at the time). It would be cool if they made these air bags with a schrader valve so I could pump them using my bike pump though, the bulb design worked OK, and is probably used since it's self limiting (hard to blow out the bag when the bulb would probably go first). To jack the slab using the air wedges, I had to wedge some large rocks under the base as I went, and pounded them in with a hammer, then pumped the shims up and wedged again with rocks, and pumped the shims some more, till the base was level. Then I backfilled under the slab with large rocks and sand and packed them in, before releasing the pressure and removing the bags and backfilling some more.
I know that sucks, and I will probably regret not getting some actual concrete mix to fill the void, but as you can see in the pics, it was late, and I was in a bit of a rush to try and wrap it up. I'm sure the pool gods will punish my laziness when I have to come back and pour a slab for this damn thing (hopefully in the off season), and probably replace the "Vari-Flow Assembly" as well. But the pool gods did get one good laugh at my expense, since I ended up having to remove some sand in order to re-position the filter - after I had leveled the slab with all the sand (but no water). I should have emptied it to start and it would have been much easier to level the slab, but this is my stoopid stuff lol.
Below are some pics I took during my recent work on this pool filter mess. The first pic shows the cracked coupler before it was glued back together (it was not fully cracked). I'll just mention that the brown start capacitor shown in the pics was mounted on the back of the pump under the back cover, and the sliver run capacitor was under it's own cover mounted laterally on the top of the motor (the run capacitor was OK, but the brown start capacitor was dead). And since I mentioned the capacitors, I'll also just say that folks gotta be very careful with capacitors. Capacitors generally and especially large ones, like the ones in the pool pump motor, hold a dangerous charge even when they are disconnected, so they must be fully discharged before handling them. I don't suggest messing with capacitors unless you know what you are doing.
Update 4/2/2023 - The superglue coupler fix had been working perfectly, but there were several other leaks which popped up since the filter repairs, and required replacing the "Vari-Flow Assembly" last month. Due to the very tight space and lack of places to do viable cuts, swapping the valve was a difficult job, but using some PVC Unions really helped (and will make future repairs simpler as well). I also had to re-repair a couple pinhole leaks in the water rotor base, which would be a real pain to replace (fortunately the fix seems to have worked).