A stubby wrench will get in there. It will require some contortion on your part however.

The upper intake manifold appears to be "sandwiched" between the valve and the tube, so no, you wouldn't be able to remove it. Keep in mind that this was originally assembled outside of the engine bay, so there's not a lot of consideration for owner/mechanics to get their hands in there.

Since you have a problem with the nut continuously loosening, you might want to check for proper assembly and sources of vibration. Is the EGR tube loose at both ends or vibrating excessively when running?