Ok, I tried doing the primary + archive rescue idea (posted above).
I booted to a minimalistic secondary snapshot. Then, I tried to update my primary from there.
It didn't work because the archive doesn't show up in the list of "source" snapshots. The instructions show a little camera icon for archives. I don't have that. I have big red cross in the tree, and it doesn't show up in the source, so it is basically useless. I wonder if it's corrupted.
Not saving my day...