I hypothesized that the DE around a hole in the fabric would show signs of erosion. So, without rinsing off the DE from the grids, I disassembled and examined each panel. As I found these eroded patches, I marked them with a Sharpie.

When I finished searching, I risnsed off the grids, leaving just the demarcations (around holes of varying sizes and shapes.) The air bleeder was in the worst condition, a 1/4" opening at the tip.

I checked and relubricated the O-rings. Then, I sewed up the air bleeder and patched the grids with low-temperature hot melt glue. The MSDS claims that the glue has very low water solubility/ extractives and is stable at pool water temperatures.

When I reassembled the grids, I noticed some of them no longer seated perfectly against the manifold because their frames were cracked. But I figured to get one more season from them before buying a new set.

After vacuuming up the DEbris (sic) at the bottom, I ran the pool pump/filter for several hours with no detectable DE in the pool/spa. I had almost forgotten what "polished" looked like.

Waste, Thanks for your advice and support.