First off many thanks to the various list members who responded both
publicly and privately you were collectively
a MAJOR help.  I'll submit a small writeup for the archives!

The oxygen sensor's "return" ground appears to proceed through the
various rusty corroded components of the exhaust system to the engine
heads and hence through the infamously hidden ground point 18 to the
ECU. ( this was probably designed during Octoberfest! )  On my engine
( with AC ), GP18 is very very well hidden under the compressor mount
and is essentially inaccessible without alot of needless component
removal. ( grrr)  Since I was able to measure about 50 ohms between
the oxy sensor case and the head, my initial concern was for this
connection.  However, after connecting this with a wire ( which I'll
leave in place for good measure), I found my problem still exists.
GP18 is combined with the wires from the hall sensor before
proceeding into the ECU wiring harness.  I cut into this grouping
just as it leaves the engine compartment and sliced an additional
ground wire from there to the same wire I had previous run.   And it
works  Yeah!    A few minutes of blowing the soot out of the exhaust
system and she's running clean.

Before I proceed down this pathway, I had also replaced temp sensor
II and cleaned many contacts and grounds, all to no avail. ( or
course not finding GP 18 hidden away like that was a major impediment
)

Chris, proud of himself.