John,
Sounds like your luck is changing for the better. Glad to hear it.

You did not mention if you took care of the engine grounds.There are several
that are critical.

How about disconnecting the ECU from the O2 sensor but leave the meter
hooked up to the O2 sensor. Drive around a bit to see if it is running rich
or lean. An adjustment to the AFM  spring could correct it.

Was the spare AFM new? If not they both could have a fatigued spring.

Do the spark plugs have the correct heat range?

I have found an Inductive Timing Light  to be a useful tool in situations
like this. Harbor Freight has them dirt cheap. Perhaps you could clamp it to
each wire, one at a time, and narrow it down to one cylinder. Then you could
test your injector theory by swapping them around.

Sounds like a tough problem, I hope your luck holds out.
Ken Lewis
http://neksiwel.20m.com/
Vanagon mods, tech info and O2 sensor help