It really sounds like a vacuum leak somewhere. I know you've done lots of diagnostics on it, but there has to be another source of unmetered air getting in there. This is similar to the idle situation my 86 had before I replaced the intake gasket and the black plastic air hose. Take your air hose off the AMM and intake and inspect it thoroughly.
Also make sure the hoses going from the throttle body area to the charcoal canister are connected at the opposite end- maybe one broke off?
The flame trap does look OK?

I've never seen a simple hand held NOx sensor.
If you absolutely cannot find anything else wrong with it, you can probably get it in spec by setting the timing to 0 degrees. It won't pass at 1000ppm? I thought it was allowed 1700ppm that year. Perhaps I'm thinking of 86-87.

Also, the engine temp is perfect. Hot engines run more efficiently. I wouldn't change a thing.
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Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K