We've only dumped a timing chain once on our recumbent RANS tandem. (Never on the upright KHS we previously rode.) The RANS has an aftermarket idler which directs the chain and is also easily adjustable for tension - much easier than the original stock idler system. In spite of the ease of adjustment, I had neglected to keep up with checking tension - resulting in the chain's jumping off on a bumpy stretch of pavement on a ride a few years ago. Thankfully, we got stopped with no problems and was able to get the chain back on with proper tension with a hex wrench. As you have found, a loose timing chain can result in much trouble.
Yes, communication is paramount for riding a tandem but surprised you didn't notice a problem before things went from bad to worse for your stoker.
My stoker has always ridden with either half-clips or, now, with clipless pedals. She has never used clipless on a single bike but no problems on our tandem.
