I've never done premium viewing for Fantasmic, so I don't know about reservations, but the number is (714) 781-DINE.
People start sitting down and grabbing up spots for the 1st sowing around 5pm. So that is a problem is you don't want to wast 4 hours of your life staking out the best possible spot. 1 hour is usually ok if you don't mind standing in a sea of standing people.
The second show is held out of Rivers of America until about 30-40 minutes before the show starts and then they let everyone in. It's a mad rush and can get quite hectic, but you can sometiemes (if you get in the crowd early) get a really good spot. Depends on which crows you want to battle.
Our revels now are ended. These our actors, As I foretold you, were all spirits and Are melted into air, into thin air: And, like the baseless fabric of this vision, The cloud-capp'd towers, the gorgeous palaces, The solemn temples, the great globe itself, Yea, all which it inherit, shall dissolve And, like this insubstantial pageant faded, Leave not a rack behind. We are such stuff As dreams are made on, and our little life Is rounded with a sleep. mycroft16 on Twitter
I've never had a problem with getting riverfront seating for the second show, even if I'm not at the front of the pack. People tend to space themselves out a little more for the second show, so it's fairly easy to slip in. Also, I've seen a few occasions where technical glitches in the first show were fixed for the second.
30 Days prior to the date you want the reservation for. Also, it really does not matter which show you go to if you have reservations, its the same show. If you go to the first show you are welcomed to wach the fireworks from there as well (and this year they will have snow at the ROA as well!), in fact the tell you it would be best if you stayed there to watch the fireworks as well.