If you're using one of the carrier trade-in options, do you have to go through all of that when trying to get one from the app, or can you just reserve it and take care of all that in-store?Yes, Friday around midnight the store inventory should be filled up.
You should check Thursday all day to look for store inventory - it depends on when the employees scan the store inventory.
When you see store availability, grab it and pay for it immediately, otherwise you have others that will get it and you may be out of luck waiting in line.
You may have to travel to further distance Apple Stores to find your trim, model, color and storage.
Also, to OP, I have already seen a device or two pop up on the apple site as being available at a store in my area when starting the preorder process, so maybe these are preorders that were canceled or otherwise became available.