What is the safest/best file format for a draft?

Upload the MS Word file to Google Docs.

See this:

https://www.laptopmag.com/articles/convert-word-doc-to-google-doc

Use the comments feature in Google Docs after you share the doc

I exchange article drafts with the other "staff" (all of us unpaid, for the past couple of years) of the on-line magazine where we share writing and editing duties in Word format. Dunno why anyone would thing a Word file is a virus carrier. If you don't use macros (and embed them in the file), it should be safe. That said, for other folks I send PDFs. I think newer versions of Word now include the capability to export to PDF. I still work mostly in Word 2003 (with the compatibility pack) because I prefer the old, classic menu structure. I team that with a freeware PDF virtual printer utility, DoPDF7. Works like a charm, and DoPDF can be set to embed fonts so that whoever views your file sees it as you saw it.

I think DoPDF is now up to version 9 (maybe beyond that, by now). I tried version 8, and I found that I didn't care for some of the [alleged] "enhancements," so I uninstalled version 8 and reverted to version 7.

For simple editing and annotating of PDFs, there a company in Germany called SoftMaker. They have a PDF viewer/editor program. The "pro" version is FlexiPDF, but they also make available a freeware version called (cleverly) FreePDF. As far as I can tell, FreePDF is probably FlexiPDF from a couple of versions back.

I guess exactly how you distribute your beta drafts is up to you, but it also kind of depends on how you plan to incorporate the edits. The very bottom line is to keep your working copy of the draft as a .docx file (or .doc, if you prefer). I actually send the .docx file to my editor, who uses track changes, and accept or reject the changes as necessary and that becomes my working file. But, I'm not sure I would want to do that with wide beta distribution. Really, you're down taking each comment and/or correction individually and incorporating it in your draft document if you do not want to have problems with file corruption.

If your copy of Word already has a macro virus, it can spread to every document you open without you being aware of it (fell afoul of that once). So yes, Word documents can be virus vectors even if you don't deliberately embed macros in them (and why would an acquaintance even be sure you know what a macro is?)

I don't think you're going to get a single format that everyone will accept and that meets your review-comment requirements. Better to figure out how to make the conversion process for several common formats as painless as possible.