Will a higher level Staff of Herding open the lower difficulty Whimsyshire?
Solution 1:
Yes it is possible to open lower difficulties of Wimsyshire with a higher level staff. In this video I open the Normal difficulty using Nightmarish Staff of Herding:
This makes us udderly pleased.
Solution 2:
Unfortunately I can't provide a video source, but YES a higher difficulty Staff of Herding will also work for lower difficulties.