My opinions/advice:
1) I'd try to get there for opening - I generally advocate that at all the parks, especially if you're only going to be there for one day. Add in the summer crowds, and getting there early means you can beat them at the bigger attractions. Hit Antarctica and the coasters early to avoid the longest lines, then enjoy the shows and other exhibits with the rest of your time.
2) Blue Horizons is probably our favorite show.
3) Loaded question. To see all the shows, ride all the rides, and spend some quality time seeing the other exhibits? All day at least. To hit the highlights, you should be able to in one day, maybe 8 hours or so. Everyone tours at a different pace, and has different tastes.
4) If you plan on spending the whole day (and eating breakfast, lunch and dinner at the park), I suggest buying the all day dining plan. For about $35 per person, you get unlimited meals at most of the parks restaurants. You also can pop into those places and get just a drink, dessert/fruit cup/etc. at any time. If you're only going to eat two meals, then it may not be worth it. One meal, definitely not worth it.
Hope this helps!