Not sure about the Grinch but I hope so! They will release more info closer to date
Hagrids does not use the virtual line during the meetup, but the line can still be longish, but not as long as during the day esp since velocicoaster will be open during the meetup as well. Almost all the other rides will be nearly walk on, or less than 10 for sure like Hulk/Rip Ride Rocket. We basically walked on to all the rides never waited more than 5 mins and that was mainly loading times. It is such a night and day difference, you will be like WOW!
Pteranodon flyers are open during the meetup but it's a lottery based ticket ride. OI will send an email closer to time that asks if you are interested in riding and to fill out the form if so. Then the do random draw and they will email you if you get a ticket to ride.
Sometimes you can ride close to closing time if they have open spots, just have to check.