I will attempt to answer these questions as best I can, being a person who's used RM2K3 for a long while, but still makes extremely horrid maps.
1. That would be known as a caterpillar system, which NOBODY WANTS TO HELP ANYBODY WITH. Sorry, but you're better off doing the type of RPG where the heroes merge into your main character.
2. As far as I know, that's completely possible, but probably just as complicated as making a caterpillar system. You would need to make a custom pause-menu system, of your own, which would remove one character, and replace them with another. All while memorizing all the peoples' points on the X-Y axis. Because basic variable algebra wins out on RPG Maker, as we ALL know.
3.See number 1. Caterpillar systems aren't easy.
4. If you're talking about MOVIE FILES, RPG Maker probably only supports AVI. Other than that, it's mostly pointless if you can make cutscenes yourself within the game.
5. That sounds pretty cool. Unfortunately, you'll A- need a ton of graphics to DO that, or B- still need to make a ton of graphics to do that. Basically, just make a battle animation for it, and make a ton of events that show the building in its new state.
6. See number 4. And YES! You can, indeed! One right after the other!!! AVI-wise.
7. Yes. Either A- Make the text on your video file with an NLE video editing program, or more commonly B- Double-click on a blank space on the map editor to make an event (alternately, double-click an event you already made) and then... double-click inside some of that that blank space below the words "event command." Finally, click on the first choice on the list you see, titled "message."
8. See number 7
9. See number 7 up to the event commands. Inside the list, click the 3rd tab, then look around. You should see "play sound effect." Picking the sound should be simple from there.
10. Typically, while playing your RPG, you'll have your talkable NPC on the same layer as your hero. Just click whatever button is the decision key, primarily enter/spacebar. Remember, no conversation will be there if you haven't scripted any words or anything.
11. Yes. But I'm not sure how you want the character to appear, so I can't elaborate on that one.
12.Yes! Under the Monsters tab in the database, just double-click inside the Behaviour section. You should see "transform" at the bottom.
Hope I remotely helped. You're asking for lots of extremely advanced stuff way too early. Take your time to get to know the program first, and read the bundled HELP FILE that you can click in the topmost PURPLE BOOK with a QUESTION MARK slightly in the rightmost CORNER. Thank you for your understanding.