"I would be interested to know if you have noted this 'borrowing' across collections in your research."
-The publisher would present a set of tunes and dances that he thought he could sell to his market as he saw it, exactly as they still do now, (how many times have you seen Harvest Home?) but with the added attraction that there were no effective copyright laws in those times.

"Could I also ask if there is a reason that you have not included in the dances attached to each of the tunes in these books."
-The dances in the 17thC collections (Playford) are already available, and the 18thC dances are a pretty random assortment of the same limited number of moves for longways, shaken about a bit for each tune according to fancy. To repeat all of them would be tedious and not very useful from a musician point of view. However, there are published collections available for those who wish to follow that side of things, and the American Library of Congress has several online.

The dance descriptions in these books are famously opaque and full of difficulty, thrown together hastily and no doubt edited to fit the available space, only slightly helped by contemporary manuals, and hindered by ambiguities and mistakes, and the fact that different masters used the same terms to mean different things. Small wonder then that Sharp is still being 'improved upon' even after 100 years.