Finding the root cause continues to elude me...let me share what I've done in a couple checks this evening and last.

Replaced the TBI base gasket, torqued to specs 12 ft lbs.
Verified the hose to the MAP holds vacuum
Verified the hose and vacuum canister hold pressure (though they slowly leak, not enough to make this idle so high)
Verified the PCV valve and hose don't have any cracks, and is in place
Verified the hose from the TBI to the EGR control unit is in place.
Verified the hose from the EGR to the EGR control unit holds vacuum

Found that the charcoal canister hose, that runs from a port on the front of the TBI, does NOT hold vacuum. I do not know if it is supposed to!

Then I tried setting the IAC.
Warmed engine up and in closed loop mode.
Turned key off, jumped the 1 & 2 terminals on the ALDL port, put key to ON for ten seconds. I have read the IAC should be buzzing at this point, but there was no noise. Unplugged IAC. Key off, jumper out. Plugged in PC, and started vehicle, IAC showed zero, and idle was still 1900. Is it possible my IAC is not working and holding it wide open? Any other checks I should try?

Pressed on the underside of the EGR valve, which dropped the idle while I held the valve open. So that doesn't appear to be a leak...

I also tried using Wd40 spray on the ports and intake gaskets but it didn't seem to have any effect. I then tried spraying it directly into the TBI, and it didn't have any effect there either. The fan is so strong at 2000 rpms that it's just dispersing the wd40 too quickly.