Speculation, but would one possible workaround be to stop using the balloon comment mechanism – since this doesn't let you work with files in different versions – and use styles instead: one style for "input from editor" and a second style for "reply from author". Use the customised ToC options to round up just the comments; use search/replace to remove all the comment threads from a finished document, or even update the styles to "hidden" to maintain an archive.

Thank you, but that complicated workaround only highlights the unworkability of Word 2013. I'm going to uninstall it and replace it as soon as possible.