You just need the ADR at Tusker House between the times indicated by the PP. We even showed up early for our 1:05 ADR last year and still got the preferred seating, so it is somewhat flexible as long as you book the ADR in that time period. A CM should bring you the voucher for the preferred seating during your meal/after your meal.
The seating is in the front of the theater, right side (facing the stage). If you sit there, a few things WILL happen behind you, but if you sit near the back of the reserved section, you will be able to see everything pretty easily.
I think it's worth it. The line for the Nemo show gets pretty long. Plus, you get lunch at Tusker House; it's a win-win.