-Do they open the doors at the same time as online drops (FootLocker, NDC, Eastbay, etc.)?
Nike recently put a ban on midnight releases here in the US. So it's usually just regular store hours.
-Would they let me buy multiple pairs?
No multiple pairs. Usually one pair per. They'll give you a ticket or wristband (depending on store) with your size on it and that's the size you'll get.
-How long would the wait be for a shoe of this type? I'm not really trying to wait overnight, or even hours for that matter.
If you don't want to wait, you probably won't get what you want. Depending on the popularity of sneakers in your area, you may have to wait hours, overnight, or possibly days.
-Is it possible to wait and then not get a pair, or is that rare?
I'm not really sure what you mean? If you're asking if you can wait in line then decide not get a pair, I suppose you could, but idk why you would wait in line for say four hours then back out after you have you're ticket and everything.
-Do people normally camp for these shoes, and then there are still pairs left later in the day anyway?
In some cases. But again, it depends on the area. Where I live (Milwaukee, WI) we just got a HOH at the beginning of the year. And since being a "sneakerhead" is a cool thing now in the US shoes will sell out almost same day. There are some stragglers usually though; in the larger and smaller sizes that will be there for a couple more days.