OK, time to close that chapter - hopefully.
MAF sensor was _not_ an issue - O2 sensors were.
Here are three samples of me warming the truck in the morning, driving to work, and idling for a while. It has highway on- and off-ramps, about 2 miles of freeway speeds, fast hillclimb from standstill, a bunch of stop-and-go driving, and losing all the altitude.
Look at short-term fuel trims: they do go down to zero, never below zero, and climb up to +100% whenever they have a chance (at idle).
Two days and even-side new O2 sensor later (I wasn't sure I was ordering a correct sensor, so I only bought one):
At the very least, the even-side short-term fuel trims go in both directions.
Two more days later, and O2 sensor purchased at a local Napa store for a 1993 Nissan Maxima (part number NGK/NTK24064), with Nissan-spec connector replaced with Rover-specific one. I also ditched the "new" MAF sensor and reinstalled the original one.
Aside from annoying tendency to start at high RPM and keep them for a long time (required a blip of throttle to clear it), things seem to work okay.
The truck passed smog check (with idle HC and both idle and 2500 rpm CO numbers a little on high side), so I can forget about it for another two years.