After a nominal review, we are going to ignore Xfce. The differences in system resource usage between Mate and Xfce are not enough to worry us. Mate provides the same functionality, probably more.
I discovered mint-meta-debian-cinnamon and mint-meta-debian-mate packages, which I think goes toward installing both desktops when starting with one Live DVD or the other. We need to experiment more with that.
Regarding conflicts, we haven't tested extensively, but thus far the only conflicts we notice are a handful of duplicates in the menus, but that might have been caused by our own newbie approach to installing both desktops. We are still learning how to use Debian tools while concurrently learning how Mint is distinct from Debian.
Overall I am finding myself comfortable with both Cinnamon and Mate, despite the two desktops being nominally different in several areas. We want most users to have the option of either desktop, although some with older hardware and the lack of 3D hardware acceleration will have Mate installed.
Mutli-boot is an option for the demo system, but ideally in the end we want to install both desktops and let users decide. For the demo we also might boot into one desktop and run a VM with the other desktop so users can more easily compare the differences.
Thanks for the help.