Glad to see you were able to get things working!
For future reference, and I'm sorry I didn't think of mentioning it before you went to all this trouble, the easy "fix" for this particular problem seems to be to just omit the colons from your chapter title text in the NCX if they're not really necessary.
No one will ever see the titles anyway unless they drag your mobi to KPreviewer, and for some reason KindleGen just seems to have a random problem with colons in the <text> sometimes.
But at least you've figured out how to deal with it in future books where you want to keep the colons around.
