1) The Pi is running Raspbian and X is always started on boot.
2) LibreOffice does not support a "loop" from command line, so even though I created a script and made it executable under /etx/X11/Xsession.d, the presentation will run once, then stop and wait for Esc to be pressed. I tried ImpressRunner, but that didn't work either.
I am looking for a solid solution, because not only am I a beginner and am learning as I go, the people I work with know less about Linux in general than I do. Some options that I considers and rejected because I don't want the even more clueless end user to have a chance to break anything are:
Converting Powerpoint to movie / jpg / whatever - Extra steps on the end user side are off the table as per the IT boss.
RDP in and start the Presentation manually - I like this idea, but xrdp did not mirror what was going on on the actual Pi desktop... IE I started the slideshow on the Windows PC's RDP Window, but the Pi screen remained the standard desktop and didn't show the Powerpoint slideshow.