I resolved my problems by installing a new O2 sensor. Used the Bosch 13275 4-wire that others on this board have mentioned. All codes cleared and have remained absent for many months.

CE light has however now come back. DM shows a code 5 (EGR). All clear at pin 8. I checked the vacuum line at front of engine and the EGR valve. Both OK. Ran a speedometer cable down the tube from EGR valve to air intake as prescribed in Steve B's article. Cable would not run through completely to end (I ran it in about 36 inches) so I imagine I have a carbon deposit build up at air intake. I plan to detach tubing and check out. Will report back.