Update:-
Now using Clip in MSC mode with Mediamonkey.
Playlists now transfer correctly without corrupting one another, however, there is a problem, where after the sync, the playlists appear to be empty on the device.
Further searching reveals many people have this empty playlist problem, due to the Sansa devices requiring relative path playlists.
So, to overcome this, after syncing, I used a text editor to modify, (search & replace), the path data from \Music\Artist\Album to …\Music\Artist\Album.
It would be good if these problems could be fixed in a future firmware or a mod within Mediamonkey to avoid this extra step.