My best estimate on Witch chronology:
Betty Buckley - Workshops
Ellen Foley - Old Globe Try-Out
Bernadette - Broadway (here it starts getting tricky)
Phylicia Rashad - Broadway (April - June)
Betsy Joslyn - Broadway (June-ish)
Cleo Laine - National Tour
Nancy Dussault - Broadway
Betsy Joslyn - National Tour
Ellen Folley - Broadway (until closing)
Honestly, much as I love Bernie, I thought both Phylicia and Betsy were far better as the Witch. And yes, Bernadette did it as a favor to Sondheim. In fact, there is an article that talks about that from the NY Times...
Fun fact, "Boom! Crunch!" was still in the show when it started previews, and somewhere during it started morphing into "Last Midnight" (and at one point was an awkward conglomeration of the two)
Article on Bernadette in Woods
http://www.youtube.com/huskcharmer