I receive an error message that want's me to disable nUI+, but little else. I two box, so it would be nice to be able to set focus on my toons, for /follow focus etc.
If I remember correctly, it's a bug with all UI's at the moment when trying to use /focus from the right click of a target frame. You should be able to use the command without any issues though.
Yes, if you select the target and then type /focus it should work fine.
I usually do that all in a macro so that left clicking the icon will make the currently selected target the focus and then right clicking the icon will cast the spell on the focus.