Well I seem to have a crude workaround, though I would still appreciate any better ideas. I changed the off command for the IR543 from two commands, light1+off, to four commands, off+off+off+off, and set the inter-device delay to 1500 from 5000. This results in four off commands being sent out over six secconds. Seems to be pretty reliable and generally turns off on the first off command. This leads me to believe that the lights1 command was participating in the failure somehow.