I don't know why there're no performances on July 24 Thur. and July 26 Sat. matinee, they could easily sell out 2 more performances.
I thought the absence of a Thursday night performance on 7/24 was probably something as mundane as the need to change out the sets from Don Q to Spartacus and make sure everything fit in that theater. The change-over from Swan Lake to Don Q, of course, takes place on Monday, 7/21, when that house is normally dark anyway. But why they are skipping the matinees on Wednesday, 7/16 and Saturday, 7/26 (especially when they are doing matinees on the other Wednesday and Saturday) is something of a mystery.
Could they add performances at this late date? When they brought Don Q to Segerstrom for a week in 2010, they reportedly brought 100 dancers with them, so they likely will have enough dancers (although perhaps not enough principals?) in NYC to add performances, if they wanted to.