Now that I've hooked up the complete length of pipe, I've had one good test burn, and then run into problems with other tests:
1. Normal trouble trying to make a cold start with a new RMH (it had no mass attached to the pipes, so it was cold even after my test firings). Bleh, lots of smoke! I've had success on some days, failure on others, so the weather and temperature differential from house to environment is the first obvious culprit. But also...
2. Leaks developed around most seams (barrel, pipes, etc.). When the fire fails at its worst, then smoke backs up through every gap it can find. I probably used a cob mortar mix that had too much silty soil, too much clay, or both. Once I cover everything with extra inches of cob, I hope the system tightens up permanently.
3. My barrel lid with fiberglass gasket and ring clamp also leaked smoke! The first gasket I tried came loose after a few firings, so I probably didn't cement and cure it properly. The second gasket with better cement did not help at all though, so perhaps one problem is that I was only hand tightening the clamp around the lid. Maybe it will be truly airtight now that I've used a wrench to tighten it.
I'd love to hear other people's experiences or advice if you've run into these same problems! I've lost plenty of sleep trying to problem-solve, stressing about the coming winter and wondering if the effort is worth it, wondering how safe I can keep my family with this smoking contraption!
Trying to remain optimistic that all will be well when the outside temps fall- then I'll pre-heat the chimney to start a good draft, and try again (with some mass in place to hopefully catch the heat and keep things warm for future firings).