Using the 88 Electrical Manual which shows O2 Heater being fed (B+) from the IGN SW (I1). Your voltage measurements show O2 Heater voltage is following the actions of the Fuel Pump Relay. Sorry if I misled you on my requested voltage measurements and sent you on a wild goose chase. I will keep this information in mind and now suspect the wiring schematic as incorrect. The most important thing here is you got B+ on the O2 Heater (which you might have already had) with engine running.
Your photo on distributor indexing looks "spot on".
REM readings look good, well within the ECU range of authority. Give REM (keep driving it) time to see if LTFT changes.