i'll try to help you with your questions
1) It is definitely not worth buying a GBA for this game is thats what you're asking. The are other reasons for buying one but not
just for animal crossing. With a GBA you are able to download your NES to the GBA to take with you somewhere, but you lose the game once you turn off the GBA so its not that great of a feature; you can also design patterns for free if you use the GBA (it normally cost bells to design a patern from those sisters). The other thing you use a GBA for is to go to your island place. That's what the person in your town was talking about, if you go to the dock with the GBA on a boat will be waiting to take you to the island. I can't really tell you about the E-reader since i don't have it.
2)In order to go to your friends village you put your memory card in slot A and your friend's in slot B the you talk to the train guy and get on the train to go to his town. Since only one person can play at a time, only one of you can be in a town at any time.
3)The code used by Tom Nook is how you would trade items with people. You go to Nook and tell him the name and the name of the town the person you're trading with is in. Then you give him the item and he says a code that you tell your friend to input to Nook. Your friend will recieve the item.
I hope that helps.