From what I've seen of footage, King Arthur at Broadway on Broadway was Mika Duncan, one of the male understudies.
I also saw Merle in her first week, and what I really loved about her was that she wasn't afraid to play and try new things with the role. And, like adamgreer said, her vocals were so great! I loved hearing Find Your Grail and Diva's Lament belted out again. I'm really looking forward to seeing how she develops over the new few months.
"Peace! The charm's wound up."
--Macbeth