Hi Eleanor,
1) Text documents - journal articles/conference items/book items/ mostly come in to our repository service as PDFs currently. That maybe because that is what we shared/make open. We convert other received content in to PDF format - as that was viewed as best format back in 2010s - consistent format across platforms, PDF viewer available free, maybe less editable/fixed. We do get content sent to us in Word - whole articles or sections/tables in multiple Word documents that we combine with image files. Physics / Maths / Stats / Computer science are likely written in LaTex/TeX and then converted for our benefit into PDFs.
2) I think any change there would be resistance - but not necessarily that much - unless that had been a bad experience with DOCX. PDFs are still editable if some one wanted to - unless security set/locked for editing. *Receive* as DOCX or LaTeX or XML, but *distribute* as PDF on demand or XML might work - like PLOS ONE or open journal systems. Would need to 'sell' why DOCX was now preferred for receipt. At a tangent, seems like legal accessibility requirements of repository content could be improved along the way, maybe?
3) Long term you would write in XML or GUI writer - that is XML/JATS underneath - like Overleaf (?), Substance (that seem to have closed down?). DOCX is XML underneath with Zip archive? Different subject disciplines are likely to adopt different approaches at different rates. Janeway open journal system team, OJS (?) and PLOS team might know about conversion libraries / preferences?
4) Sorry, no. The Plan S promotion of JATS - seem strange when I read it a while ago - more focus on publishing industry rather than current repository current usage.
Apple Pages / Google Doc - not detecting this used a lot or sent our way.
Good that someone is consider implementation of Plan S in repositories.
Linds