Unless there is an illness, injury, act of nature, etc. JKR is 100% going to be there on the 18th. I know it for a fact (although I can't say how I know, sorry, I know how annoying it is when people do that).
I also know that barring illness, injury, acts of nature, etc. The trio, Harry, Ron and Hermione, and Draco will be present (and I suspect Dumbledore and Hagrid as well), and here's why I say this:
When actors sign contracts for projects of this size, be it movie or theme park ride, there is ALWAYS a clause in that contract that says they have to do press junkets and media events for that project. The only exception is generally if there is a contract that pre-dates this project's contract that has scheduling conflicts and the party that contract was made with is unwilling to let the actor off for a couple of days.
In the case of the above mentioned actors their contracts with Universal would have been co-negotiated with Warner Bros. and Heyday Films. And since their current scheduling conflict is their project with WB and Heyday (aka Deathly Hallows), WB and Heyday would be fully prepared to let them off for a few days to fulfill their contractual obligations with Universal. I can't speak to whether Gambon and Coltrain, who also filmed for this ride, will be obligated to do the media events as well, because most of the adults in the HP franchise only film during select months, and generally have lots of other projects going on in between their HP filming. Radcliffe, Watson and Grint however are generally exclusive to HP when they are filming because they are in more scenes.
As to whether guests will be able to enter WWoHP on the 18th, I don't think anyone should expect that their breakfast reservations or something some phone operator has said should guarentee it. At least not in the morning or during the official "ceremony". We know the Today show and Ellen are likely to be filming that morning, they both had contests, etc (remember Ellen's chosen two are going back for the big ceremony with her, and Today show has the classroom of kids). When they are filming things like that, they don't generally want a lot of people wandering around behind the reporters or, heaven forbid, in front of the cameras, so there will be roped off areas. And when the actors and JKR are there, they are going to keep the unwashed masses away for security reasons.
I hope they honor breakfast ressies that were made months before this date was announced, but don't be surprised to get an email or phone call either telling you to reschedule the day, or telling you to get to the Three Broomsticks by some round about method that keeps you away from the actors/press. I'm not trying to start an arguement here, I'm just suggesting you be prepared for anything on the 18th, but don't expect to rub elbows with JKR.
