QueenAlice said: " Glenn Close missed a number of performances 20 years ago, so I find it very hard to believe that she will be able to do the show until June without calling out at least a few times. I too was a little surprised that they don't have somebody covering her who is a little closer to her age, particularly since she is now significantly older than the character she is playing and that now must be a part of the conceit of this production. I suppose it's possible they could bring in another woman to be a standby, leaving Nancy Anderson to just be an internal cover. Karen Mason, who I saw go on for Glenn in 1994, could still do it."
If I remember correctly, 20 years ago, Glenn missed two weeks for the infamous "Vacation-gate" that was announced. Other than that, I don't think she missed a great number during her 9 month run. And last year during the 5 week run, she missed a few performances which made news because of the extremely limited run and the whole thing being almost completely sold out (so it was hard to refund/get new tickets) But I think that's why the 7 performances per week was done so that once it opens, Glenn will have 2 full days/3 nights of rest between performances so she can recover and be prepped (might encourage people to see the Wednesday performances as she'll be fully rested for those!)
As for Karen Mason - I wondered the same thing - would she be up for being an alternate/understudy? Is she too big for that role at this point? Having seen her twice in the original production, her voice is AMAZING - and I would go back just to hear her singing with that orchestra.