I've got my custom worldspace working correctly as a child of Tamriel, but now need to add some additional large mountains to the valley. The problem I'm having is that in the CK I can't pan far out and still see the mountain meshes - they disappear once I get a cell or two away. I've tried various settings in the preferences, but nothing seems to help so far.

What is the best way to work with mountains, cliffs, or other large meshes at a distance so you can see it all?

First you need to make sure the meshes your placing have LOD attached to their base objects and the reference has is Full LOD toggled. Also not 100% sure but 99% that the CK uses the same LOD meshes as the game so they will need to be generated before you can see them. If you got all that covered then you'll need to up the UgridsToLoad setting in your ini.