I can think of a few ways of doing this but my automation experience is VERy limited!

1. Change your word object so they can't close it normally.
2. Check that your object is still valid by executing a harmless function (workbooks open etc) and trapping any error. (0 out of 10 for style)
3. Check if the process is running

I would be temped to just open a empty doc first (so the program is in memory) then a new one for each document you want them to view and then close only the review document until the end when you can close the other one. If you do this by opening the document and not word (thus letting windows handle words start up because of the association it would be pretty academic if they have closed it.

You would also be able to re-open your dummy doc if you have an error when you close the last review document (if you needed to).

I am quite interested in what someone with more automation knowledge suggests.