Ok, what exactly are you trying to do with thunar-archive-plugin and what exactly is happening?
Yes, xarchiver is the "preferred" application for opening archives under Xfce. You're sure you don't have archivers/xarchive instead of archivers/xarchiver? These are two different programs.
As for MIME, don't you have /usr/local/share/applications/xarchiver.desktop?
You may also create a .desktop file in ~/.local/share/applications and fill defaults.list accordingly (mime/type=some_name.desktop).