I am testing Nextgen Gallery, and it's really good. Thanks for sharing your work.
However, I think that the sorting could be much more flexible:
1. A global "sort-by" option doesn't make sense. At least it doesn't for me: some galleries must be sorted by PID, others by filename, others by hand. So "sort-by" should be gallery specific
2. when the sort-by of a gallery is set to "custom", it should still be possible to sort it "in place", ie. "Sort this gallery by PID/name/... now!". Right now one has to do it manually
3. When the pictures come from different sources (with different naming conventions), the sorting features of NGG are of no use. And it's no fun to sort 40 pictures manually... So sorting by EXIF creation date would be great.