The Grand One is an amazing boat. It is absolutely beautiful and remarkably larger than it looks. You would probably have to eat downstairs or upstairs in the back. There may be a minimum time, but I don't know. I would suggest a two hour tour with dinner and then wishes... but watch it from the front of the boat outside (there is non slip stuff so you can sit down). It is breathtaking from there.
That said, we have only done the wishes cruise and haven't done dinner. I don't know about private dining but I'm sure a quick call would give you a ton of locations.