1. No you can't
2. It has a better storyline in my opinion than R/S. You can catch Rayquaza pre-Elite 4. Can catch both Kyogre and Groudon. You get to choose what Lati you catch. There is a whole new area in the Safari Zone which has G/S/C Pokemon in it. A whole new battling challenge called the "Battle Frontier".
3. Not unless they are from G/S/C or already in the HoennDex. If not then the answer is no.
Next time you have a question about Pokemon Emerald ask it in R/S/E Simple Questions thread. The link to this thread is in my sig.
This forum is for general Pokemon discussion, but if you go up a couple you will get to specific Pokemon Handheld games.
I will request that this be moved there, and I will lock it when it arrives.
