The only instance of this that I know of (and I am sure there are others, I'm just not aware of them) is the way Bette Midler has done her stints in Hello, Dolly! In that case, there were always large signs outside the Shubert that made clear that Donna Murphy was going on that evening (Tuesdays), as well as during Bette's scheduled vacation period last year. When you went to order on the Telecharge website, there was also a red disclaimer at the top of where you picked your ticket that said Donna Murphy would be playing Dolly on any Tuesday evening show (except, naturally, the added Tuesday evening show on 12/26/17).
The same is true for the upcoming run: Telecharge has a red disclaimer on the seat selection page notifying potential buyers that Donna Murphy is going on as Dolly during Sunday matinees. The official show website also noted dates Donna would be on instead of Bette with an asterisk and, if memory serves, they were in a different color during the 2017 run.
To me, that seems like the right way to handle it, and for as much grief as Rudin has gotten over the years, I think it's a marvelous way to ensure people have the information they need. Some people will still miss the disclaimer and might end up disappointed, but in my opinion, you should read the site carefully when ordering tickets as pricey as these (though the disclaimer could stand to, at least, be bolded)!
As to your second question, I think that as long as the information is disclosed in some form, that's enough. While the theatre/ticketing website/production team should make every effort to get information about a star's schedule out there, buyers have at least some obligation to double-check before completing their purchase, especially in today's age where info is basically a social media post away, if not a quick Google search.
Not sure on your third question, but I always assumed it was above-the-title on the Playbill (which usually is the same as the marquee, unless there are more than a couple major stars involved).