thanks a lot for your suggestions. I'll test the trick with the DITA-Map "Print ready" styles. Very good idea.
Maybe that fits my needs already.
I have two more transformation scenarios on my todo list, this one would have a lower prio. Maybe I come back to this later on. I'd prefer a PDF-based solution, because you'd be able to communicate within the PDF and you'd be able to archive the file. Probably it would be much easier to render if the comments are not used across element borders. It would be nice, if Oxygen would have an option to restrict the usage of the elements by config to avoid issues later on, but that's just an intellectual game.
I do not want to use another commercial renderer, so Prince is not an option for us.
Maybe someone else has an answer to my second question.
I wish you a merry X-mas!