It's hard to say, it all depends on how many people are pre-ordering. I'm sure Apple allocates a certain amount of stock to pre-orders guaranteed by the 24th. Once that number is reached they have to push the delivery date further. I would think that if you get your order in on the 15th you should be ok. Wait until the 16th or 17th then it gets dicey.
Yes, of course. If history is any guide Apple will tell you when you can no longer order for launch day delivery, but that probably will not happen until the 20th. Depends on whether the iPhones ship from China or the US.