Gluing the top edge of the keycap in place seems to have worked for now.
I slipped a shim of greased/waxed oven paper underneath the top half of the scissor mechanism to raise it up, and to protect the PCB underneath.
I put a small blob of glue on both of the clips at the top of the keycap, with a thin line of glue between them.
I hooked the bottom half of the keycap in place, then pressed down on the top half until the key was fully depressed.
I placed a weight on the key to hold it down while the glue dried.
After the glue dried, I had to use the paper shim to lift the key back up.
After removing the shim, the key appears to function normally.
I’ll probably buy the v2 keyboard when it’s released, in case any other keycaps are wearing out, but at least I have a working laptop in the meantime.