We backfilled with all the dirt that we had previously spent days hauling away, and that has given it some stability. We never did figure out how to use mortar to affix pieces along the way, so we skipped that part entirely.
It's done now. The Advil is back in the cabinet, my knees are healing over.
Apparently I bought a pallet of "wallstone", which are fieldstones. They are irregular and not as flat and even as flagstones, and that explains a lot of my frustration -- I kept envisioning a more fitted, easier to stack, low and tidy wall, but the materials wanted to be a chunky rough wall.
It does what I want -- holds back the slope of dirt from the raised garden, offers a sense of enclosure to the space behind it, defines the edge of the pavers.
Eventually the young smokebush in the garden above the wall will become large and fill the open space. I transplanted some lambsear that had been struggling in too much shade to the corner just to fill in a little.
I am very pleased, but I do have a few critiques.
I'll have to figure out how to hold back those few inches of dirt, but wasn't that what the wall was supposed to do?
And the right side ends in an odd curve under the dwarf blue spruce. I think I can fix that, but again I am puzzled why the wall doesn't go all the way to the corner.
There are other defects that only the builder would see, so I'll spare you those, and try not to see them myself.
Overall, for a first time attempt, using the wrong kind of rocks, this came out great.