EDIT: This article was taken from Blizz. "On Patch day, players will be able open the Battle.net desktop app and choose which game you'd like to play, either Burning Crusade Classic or WoW Classic. Your current characters will appear on WOW Classic TBC Gold both types of games under the name of your previous realm. After you've selected a game to play, you'll choose a character to play and then confirm your choice. If you decide you'd like to play as a character in both game types then you can make use of an optional service to gain access to the cloned characters in both game types. All you need to do is log into the game as usual and begin playing.

Yes, as the copy is done before release. Classic has all the characters, which means there's no copy delay if you choose Classic. TBC can cause delays, simply because it's not immediate. It could cause more characters being copied at the same time. This could lead to the system slowing down.

Bots are likely to utilize the boost, which makes it more difficult for buy WOW TBC Gold bots. But if bots were removed I'm not sure the boost would cause much of an issue. I have 5 friends coming back to classic because we all like tbc, and they are only joining because they can boost past the vanilla levelling content.