Not that I am saying I know anything about what Bundy does when she isn't at work. But, could it be that she doesn't take the best care of her voice when she is on her off time?
I am just wondering if she took better care of herself when she wasn't working might she be able to hit that note every night?
Also, producers try to avoid prerecording things when they have to. The only reason why the title song in Phantom is prerecorded is because you have four actors on that stage the double for Christine and The Phantom and the regular two. The regular Christine and Phantom are climbing up a latter and down a ramp to create the image of them going down into the lake under the opera house. And, then they walk under the stage to where the boat is and get on. That is why they prerecord that song. And, I have heard a few actresses who played christine who said that they could go on 8 times a week without a problem. But, I don't think anyone is going to say no to two paid nights off.
And, if memory serves correctly in the London revival of Evita they didn't use an alternate.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll