Well that was a disaster.
• Only the iPhone version of the app was live, at 5:01 am. The iPad version and the website were still "getting things ready" for about 5 minutes past 5.
• You cannot use the iPhone app to purchase an installment plan iPhone on your wireless account that isn't the phone you're physically holding. It defaults to the carrier and device you're using, with no way to specify another line on your plan. EDIT: I should clarify, the second option is broken for me. I put in the other devices phone number, the last four of my SSN, and get "An error has occurred" or "Your session has timed out" depending on if I try it on my phone or my Mac.
• Apple will not ship an iPhone to any address on your AT&T account other than the one specified on the account. Sucks when you're trying to get your brother a new iPhone and he lives 400 miles away. So you think you'll try to order from the wireless carrier directly, because they will ship to another address. Only problem is AT&T's website is about as useful as ever and is completely broken. Still barely functioning as I type this at 5:48 am.
In the end I managed to snag my phone (12 Pro, 256 GB, Pacific Blue, full price) at 5:02 am because I had it pre-saved. I got an October 23 arrival date. I ended up getting my brother's phone (12 Pro, 128 GB, Pacific Blue, AT&T installment plan) around 5:23 am from Apple's website. That has a 7 - 10 business days shipping estimate, and I have to ship it to him once it arrives at my house.