When exporting multi level numbering to RTF, numbering disappears.

"My colleagues do not have acess to the full version of acrobat so can not edit PDFs"

Your colleagues must not edit the PDF, they should use the comment tool with Acrobat Reader, then you import the PDF comments in your INDD layout.

Otherwise you can Share and Collaborate:

https://helpx.adobe.com/indesign/using/share-and-collaborate.html

Or Share for review:

https://helpx.adobe.com/indesign/using/share-for-review.html

Welcome to the 21th century! 😉

It's been many years - almost twenty - but I've been in your shoes before. Before translation tools had the ability to process IDML files, I exported and re-imported carefully styled RTF files as a daily workflow practice. The paragraph styling has to be perfect in order for it to work. Even so, the RTF export filter isn't perfect, but if all of your e.g. third-level paragraph numbering is perfectly styled, then you can open the RTF in Word and add auto-numbering to the style there.  As you observe, the tabs and indents and etc. come through the RTF export process just fine. Then, when your contributors have finished their edits, you re-import their RTFs using style mapping, so that the InDesign styles are applied to the RTF contents upon import. When it works, it works really well; just re-import the RTF post edits, allow InDesign style mapping to clean out all the MS Word crud, and then you can clear all overrides with a clear conscience. 

That's how I did it, back at the beginning of my career. Today, almost any other method would be superior. Bob has already covered all of the good solutions. There are some potential mediocre multi-step conversion processes  that haven't been mentioned yet. Perhaps InDesign -> PDF -> Word .docx should be classed as actually worse than what you're doing, but I've successfully used that workflow before, as well.  But "using Wordsflow" and "post-export RTF correction" seem like your most likely solutions.

What's wrong with PDF?