I've always been seated about 15-30 minutes past my reservation time at Tusker House. It's a buffet and a character meet so sometimes they have slower turn over. Personally, I would feel too rushed with your time frame. But you might be able to do it. And I think the seating area for Rivers of Light is over in Asia but I could be wrong. If that's true then you also have to consider walking time from Tusker House.