Too much going on. I'm losing track here.
Warps #9 and 10: backstrap success!
Not woven on a backstrap loom, though, as I'm not yet that competent. The backstrap itself is rep weave, 5/2 cotton for the warp. Ditto the end loops. I cut and sewed the loops too short at first, but just repaired them with inserts.
Now, I just need to wind a warp for honest-to-God backstrap weaving!