Your title shows "HD" dvd. Are you actually working with HD-DVD's? (a discontinued, obsolete format now)
If you are trying to edit normal dvd video, you should get the trial of VideoReDo TV Suite or Womble Mpeg Video Wizard DVD (horrible name). Both are designed to work with mpeg2 found in dvd video and will edit this format with the least amount of re-encoding possible.
The trials are fully functional, so you should be able to test them and see if they meet your needs.