Ack, I misunderstood you. I've retreated to a version with car/cdr manipulation; looks like there's no way around it (it's still simpler than the original, but that just makes me wonder what bugs remain)
I haven't tested it, but I don't see any bugs from here.
I'm a bit surprised you broke up the (if branches ...) condition into two places like that, though. I think it would have been simpler just to replace "(or then more-branches)" with "branches". ^^
