All servers currently running will be TBC servers from Megaomgchen's blog

All servers currently running will be TBC servers. New Vanilla versions of those servers will be made available. You get the choice of WOW TBC Classic Gold activating the character on the TBC server or the Vanilla version. If you wish to activate the identical character on both servers, you'll must pay.

Edit: This article is a reprint of Blizz. On Patch day, you'll be able to access the Battle.net desktop and select the game you wish to play: WoW Classic or Burning Crusade Classic. Your existing characters will show on both game types under the name of your previous realm. Once you've chosen a game to play, you'll pick a character you wish to play and confirm your selection. If you decide that you'd like to play as your character in both types of games it is possible to avail a paid option that allows you to play the cloned character in both game types. All you have to do is login as usual and play."

Because the copy is finished before release, yes. Classic contains all the characters, therefore there is no delay for copying when you choose Classic. If you choose TBC the copy may cause downtime (just because its not instant) and imagine a lot of people's characters being copied at the same time which could slow the system down.

Bots are likely to take advantage of the boost, making it even more challenging for bots. If bots were managed, the boost wouldn't be a problem. I have 5 friends coming to classic, as we all love tbc and only due to the fact that they can increase their levels beyond the vanilla levelling content.

Every character we have is in Classic already. They certainly don't want down time for anyone who is going to TBC. The entire database will be copied prior to launch. TBC will flip the "active bit" on the TBC database to true and the Classic bit to cheap Burning Crusade Classic Gold false if you select TBC. Then you can pay $35 for them to flip the Classic bit back.