As it turned out, I spent a fair bit of time fiddling with adjustments on the new-ish rig. Seems that derailleur hangars sometimes bend when you try bashing them with broken limbs. Who knew? So, I found an appropriate branch to use as a makeshift repair stand, applied an "equal and opposite force", and re-tuned the cable to get back to a crisp interchange of gears once more.
Then a badly behaving brake made for more fettling, although there wasn't a whole lot one can do with a hydraulic set up in the field. Meh.... I ended up eating into a big chunk of my ride time allotment with all this mechanical grubbing, but that's the way it goes sometimes.
At least I got sum skilz at mechanickin'!