Hi Chris,
For what it is worth I'm a big believer in not combining tracked shots and untracked landscape shots. As you have found out if you have any terrestrial elements in your tracked shots they will blur and if they are a long sequence of shots (like yours) then that blur can really take up a large portion of the image and make it almost impossible to blend back in your untracked images. I suggest that if you want to use this technique, you limit your sky exposures such that your terrestrial elements only move a little bit. You then should be able to paste your untracked foregrounds over the top without too much trouble.
or, you could do what I do and take single fixed tripod images keeping my exposures short enough that the stars don't move noticeably. It is of course a constant trade off between noise and exposure, but it's what I love to do. Couple of examples:
http://www.iceinspace.com.au/forum/s...d.php?t=139374
and
http://www.iceinspace.com.au/forum/s...d.php?t=142651
Kind Regards
Greg