I am not sure if this will help, but this is the voodo applied by KDE folk to use Dolphin instead of Nautilus.
In the console, see what this will show:
Most probably, it will show:
cat /usr/share/applications/defaults.list | grep inode
Now, open the file in your editor (in this example, /usr/share/applications/nautilus.desktop) as root, and modify the section
I would be curious if this actually works for somebody