Progress: thanks.
I've downloaded LibreOffice and opened the .docx with it. The comments and edits show up, though the visualization is a bit off. (The page numbers are jammed up against the text.) I can edit the LibreOffice version, and can see the revisions tracked.
When I close the document, it saves as a .odt file. This cannot be opened by Pages (and neither can the other formats available on LibreOffice). When I move the file to a PC, MS Word does open the .odt file. But now the changes I've made are not tracked: they simply appear as text.
So we are closer, but not there yet. My coauthors (who use Word) will not be able to see the changes I've made as tracked.
I'm wondering if the original problem can be approached from another direction: since short .docx documents (up to ~5 pp) can be opened in Pages, including the markups, perhaps the crashing is due to a buffer overflowing. Is there a way to modify Pages so that the problematic buffer is larger? Surely I am not the first person to have experienced this problem. Does Apple pay attention to this discussion forum, or is there another way I can seek help from them?
Thanks again for your assistance. It was certainly interesting to learn about LibreOffice.