Well, the new manifold is in and relief was noticed - for awhile.
Seems that the problem only occurs when the pool calls for solar.
At that point the pump bumps up to accommodate the water going to the roof. I get a brief surge of DE into the pool followed by a regular particulate stream - as far as I can tell only during the solar cycle. There are no discernable leaks visible and the level hasn't changed much. I get some air build-up in the filter, noticeable when I turn the air relief valve. I'm throwing my hands up at this now.
So it's time to retire the DE filter, imo. There are enough good cartridge alternatives on the market.
On that note, I'm looking at (among others) a Pentair Clean & Clear 420. Reason is it has the same footprint as the FNS+ so maybe I can get by with simply undoing the unions and replacing the filter. Otherwise, there's mucho plumbing to be done.
So the question is: can I leave the Multiport valve in place with the 420? I know I don't need the functionally of a multiport, but can I just leave it on Filter mode? That would make a huge difference in the plumbing. If I need to re-plumb, I'll go with a pre-filter/cartridge combo from Circupool and try to find a good plumber.