Can I play a copied DVD in my Xbox? Without modding the xbox itself or
re-copying the movies? I originally used Nero to copy them and a
compression utility to ensure they would fit on a DVD +R disk.

no you can't play copied dvds on an unmodded xbox.

You can, however, play copied xbox games without even oppening your
xbox case, it's called "Softmod" here is a link:
http://forums.afterdawn.com/thread_view.cfm/218495

If you want a good xbox mod chip, i seggest the Xecuter3, I have one
and it is great, you can turn it off by holding the power button in
when you turn it on and it will start in the origional xbox bios (so
you can play live without them knowing you have a modded xbox)... it
usually cost between $60-$80USD.

-ALL