I don't have time to try and tackle this, sadly, but I did notice something potentially useful:
Part of your problem could actually be a bug in update-manager.
update-manager tries to use the org.gnome.PowerManager interface to inhibit sleep before it tries the freedesktop standard (org.freedesktop.PowerManagement) that xfce-power-manager uses. This could start gnome-power-manager unnecessarily.
That may not fix your problem completely, but it's a start.
As posted earlier, you should be able to just get rid of gnome-power-manager to fix this problem.
I don't have time to try and tackle this, sadly, but I did notice something potentially useful:
Part of your problem could actually be a bug in update-manager. PowerManager interface to inhibit sleep before it tries the freedesktop standard (org.freedeskto p.PowerManageme nt) that xfce-power-manager uses. This could start gnome-power-manager unnecessarily.
update-manager tries to use the org.gnome.
That may not fix your problem completely, but it's a start.
As posted earlier, you should be able to just get rid of gnome-power-manager to fix this problem.