Wow sir, you have done most everything I had to do on my 1990 240DL wagon.
My best suggestion at this point is to check these next 3 things.
First, and this may sound strange but it worked on my car when I had your conditions. Clear the trouble codes on the self-diagnostics. Then, disconnect the negative battery lead and disconnect and reconnect the instrument gauge cluster. Lastly, re-seat, re-clean or replace the fuses.

Oh, just remembered two more things to check. I didn't notice in your post a mention of cleaning or replacing the flame trap/pcv. Mine idled high like yours till I cleaned out this. The idle should be close to 775 rpms according the the Bentley Volvo service manual for your model year 1989.

Second, is the alternator supplying enough voltage back to the battery and thus the coil and wires. The wiring from the alt to the battery/gauge cluster runs under the engine and often gets frayed causing it to short to ground and cause
the crazy miss,shakes,rough idles.

Best wishes,
Brandon in Franklin, TN