I saw it twice too & it worked wonderfully. Except for the second time during 'You Can't Stop the Beat' a computer error message popped up on the screen.
She did sing the last note in "Thank Goodness" today. I made sure to pay special attention, and actually for a second I thought she might not do it. I'm not sure how it would even work if she didn't sing it, but she did
Someone else would sing it for her, which may have happened without you noticing. It's the Glinda understudy who is on the bridge during the song. Kendra Kassebaum does not sing the note at all.
Mackey started in a regional sit down as the understudy, working her way up to the role,
I say nay nay. She was only understudy for less than 2 months before the producers bumped her up to Chicago. Unlike say Katie Adams who has stood by for a lot of the Broadway/tour runs & still has no principle role to show for it.