Sorry, but this is quite confusing, I am not sure what is your problem/what do you need. In general - read tutorials in Getting Started section. I am quite sure that most of your questions are answered there, especially in first parts of those tuts. Quite a lot of questions are asked by people who didn't read those tutorials and they, in fact, just waste both their and our time. Feel free to ask if you will still be unsure about something after reading those tuts - I will gladly help.
From whenever? uTorrent for instance? If you don't have a game, just google&download it, there is no special magic about that.
You have probably no way how to really force players to actually use your launcher. They may just run WoW.exe and skip downloading patches, and that might be problem for either them or you. Maybe you will find some.
I am not sure if TrinityCore supports anyhting like that but most likely not. TrinityCore is server emulator, but not Blizzard super-game-client-download-hosting-derver copy. As far as I know, players playing on private servers have to download the whole game on their own, with all data. Like I said before - uTorrent is fine for such things. For instance.
wotlk dont support this. if yoj deleted yoir data folded you must reinstall and update wow to 335a with self downloaded patches. or download some full wow folder from torrent.