It's too often overlooked that HTML is an acceptable* office file format.
Suppose that rather than a docx, you handed them what we might call a "doch": a self-contained (no external resources) .html file that includes more or less what you'd get by doing "Save as HTML" in a traditional office suite. This would work well enough on their end, no? They should still be able to double click to open it. The difference being that since the W3C/WHATWG specs involve a whole programming system supported by the browser, then you could also script a lightweight sidebar that they could use to add their annotations. The positive would be that, since you control the implementation and therefore the experience, you could (a) take the necessary steps to ensure that the alterations they're making follow a consistent format, and (b) can be more easily ingested by your tooling in a way that's compatible with your preferred workflow.
* close enough to cover the 80% of features where it matters (e.g. for media not meant for print)
Suppose that rather than a docx, you handed them what we might call a "doch": a self-contained (no external resources) .html file that includes more or less what you'd get by doing "Save as HTML" in a traditional office suite. This would work well enough on their end, no? They should still be able to double click to open it. The difference being that since the W3C/WHATWG specs involve a whole programming system supported by the browser, then you could also script a lightweight sidebar that they could use to add their annotations. The positive would be that, since you control the implementation and therefore the experience, you could (a) take the necessary steps to ensure that the alterations they're making follow a consistent format, and (b) can be more easily ingested by your tooling in a way that's compatible with your preferred workflow.
* close enough to cover the 80% of features where it matters (e.g. for media not meant for print)