If an iPhone has its wifi option turned on and it detects a wifi network that it can use, it will use that wifi network when possible instead of using your cellular data.
You do not have to switch it manually, although generally you need to choose the wifi network one time (and possible enter a password or go through other authentication) before the iPhone will use that specific wifi network automatically when it is available.
Some apps create network traffic all the time. Some do not. If an app is not running at all, it can't use any data, of course. Note that an app does not have to be in the foreground to be running.
You're going to have to investigate the specific pros and cons of ordering from Apple vs Verizon yourself. Only you know your particular situation (patience level, proximity to an Apple Store vs a Verizon store, etc.). At this time, no one can pre-order an iPhone 5; they have not been announced and as of this moment they do not exist.
By the way, if you're worried about using cellular data, or if you know you're right up against your limit and don't want to go over, you can turn off cellular data completely within the Settings app.
Unless you're streaming music or net radio or something similar, the amount of data used by apps while the screen is locked is generally no more than a tiny trickle. If you're not streaming, you can generally ignore the fact that your phone consumes a bit of your cellular data allowance when you're not actively using it.