the one thing with replacement rings is that often, one can purchase a whole new crankset for not much more than just new rings. I may be wrong on that, but especially if a new crankset comes with a new bb, which often you see they do, then often the difference isnt that much, or worth it in the end.

I was lucky and found some used , but new essentially, replacement 32 and 22 rings for my XT 44/32/22, but that was complete luck. I tend to keep on top of drivetrain maintenance, cleaning and measuring chain length, so expect to go a long time before I need to replace those rings--at which time I may have forgotten where the bought ones are!

my past experience with changing out granny gear chainrings for smaller ones is that so far, I've never had to make any fd changes, the triple fd generally have had enough shape of the cage to work fine with smaller rings, ie going from a 28 to a 24 back on my old 7 speed tourer which I did in maybe 91 or 92, newer bikes going from 30 to 28 or 26 (9 speed bikes) so I suspect changing the 26 to a 22 will work fine right away.

yes, with the existing chain as is, one shouldnt use the 22 front with the smallest cogs in the back, but as always, the worst thing that can happen is a bit of chain rub on a pulley, so no big deal, and anyway, it's easy to just not go past half or three quarters of the cassette in the granny gear anyway.
The only thing with just this change is that when you shift up to the 36 from the 22, with the 14 tooth difference between them, you have to shift down two gears at the back, but not really a hardship, and especially with trigger shifters, its a matter of "snick snick". Its the same with one of my bikes when I changed the 50/39/30 to 50/39/26, I generally do two downshifts when I go from the 26 to the 39 given the 13t difference--but with sti shifters, its as simple as a single sweep of the shifter and two gears are done.

overall, this is why jumps between chainrings of 10 or so teeth is nicer, as the transitions are smoother with less rear shifting needed, but in the end, changing the granny ring is still the easiest and cheapest solution--BUT I still say that a mtb triple would be more suited for this fellow if he is really going to have such a heavy load.

at the extreme end of low gearing, your comment on the 24/34 and climbing walls makes me always think of the very few times with my troll 22/34 combo (that gives 16.7 gear inches with the 50mm tires on it), that I had to stand in first gear to get up hills, paved ones. One time I had to stop every minute or two to give my heart a break, it was just not doable for more than that short time, I was at my max heartbeats whatever that is, but this was after thousands of kilometres of riding in predominantly mountains day after day, so I was in very good climbing shape.

The gradient must have been over 20%, but I realize that this is particular to Latin America--BUT I would add that there have been countless times here in Canada, or in France even, where short little steep bits have made me grateful for that 22/34 combo, and just nice to be able to be seated and NOT put a big amount of torque into your knees, even if just for 10 seconds.