I'm having trouble with the plied threads catching on each other, but it's impossible for me to diagnose and fix without being able to lock the shed open and walk around the loom. It was a simple fix. I just needed to make a peg on a chain and drill a hole in the dobby box so that the new peg will lock the dobby arm down and hold the shed open.
And here are the tricks to drilling a clean hole: clamped blocks and guide holes. First, I marked the location and drilled a tiny hole in the dobby box.
Then I drilled matching holes in two blocks of wood and used a shibori needle to line up the three holes while I securely clamped the blocks into place. These blocks keep the edge of the hole from splintering when the drill bit enters or exits the wood.
And here's the result: a new peg on a chain and a hole to insert it.
Here's one new view of some thread problems in the shed. It'll really help me to be able to fix them in the best way possible if I can stand up from my bench and get at these things from multiple angles.