Those should all be open. Waitress and School of Rock are doing very well at the box office and Holiday Inn is pretty much guaranteed to run as long isn't announced to be, because it's playing at the Roundabout (i.e. a subscription based non-profit theater)
While I don't know, I don't think Great Comet will be *that* hard to get tickets to, especially if you're okay with paying full price in advance. Yes, it has Josh Groban in it, but the show itself isn't super commercial. It should do okay, but I highly doubt it will be impossible to get (full-price) tickets. Discounts or rush or whatever might be a different matter... But it's not gonna be Hamilton in terms of the ticket craziness at all.