I think everything has settled on /usr/share/appdata by now. xfce4-appfinder.git#master:data/Makefile.am still uses "appdatadir = $(datadir)/metainfo", which should be replaced with "$(datadir)/appdata".
Hmm, I think it's the other way around, things were supposed to be installed in appdata but then came AppStream, please see Bug #14624.
Yes, see bug#15341