It's dinner at Tusker House and a ticket in to a "reserved seating area" in the RoL theater. So yes, like Fantasmic. It doesn't actually reserve you a seat, just allows you to get in to an area of seating that is otherwise controlled and guaranteed for those who are taking part in a dining package that night. So this will definitely achieve that. Otherwise, if you don't want to dine at/pay for Tusker House, you should be able to get regular seating as long as you arrive maybe 30-45 minutes early. 2nd shows tend to be less crowded as well.
Not sure about your booking issues. I had issues trying to modify the time of my RoL dining reservation on the app/website, but was able to book on the website. If you call Dining, I'm sure they can sort things out for you if you're trying to make a change.
