Well, I don't have a globbal keybinding for ctrl+alt+u, so I don't think there is a conflict.
Some key combinations don't work for some mysterious reason - I haven't figured out why.
This is currently set to <primary><alt>u. I guess <primary> is the denotation of the "Apple" key (labelled command on the Apple keyboard)?
Yes. To override it to something else, just double-click the line (or press the Change button below) and enter the key combination you want to use.