1) Yes, main building.
2) No need to go to the main building upon arrival (except for its awesomeness). The one exception is that if you arrive after 10PM, there will be no cast members in the lobby of the villas, so you will need to check in at the main building.
3) Just mosey over. There is a covered walkway. If you do advance check-in and have your magic bands, you may get a text message so you can go straight to your room without checking in. However, I usually do check in to make sure that the Magic Band room charge privileges are set up.
4) Yes, that should give you plenty of time. When the fireworks start, pretty much everybody will go out on the porch to watch. The music is piped in. It is a great place to watch the fireworks. It seems like our reservation was for 7PM, but that night the fireworks were at 9PM. Also, at some point the Wishes fireworks show is being retired, and they have not announced the replacement. It could be another fireworks show, or it could be something with drones like at Disney Springs. Just check to make sure that Wishes is still occurring for your dates.