I'm sure there won't be any shortages on any of the models after November. It's just the first two months that make it hard to get. It'd be really surprising if they can't keep any Plus models in stock in 2015.
Seems they've done a pretty good job ramping up production for the 6. Even after all the preorders it's still only pushed back 7-10 days if you ordered now. The 6 Plus not so much but given all rumored production difficulties with that model it isn't really unexpected. Really depends on which one you want. By November you should be able to walk in and get whatever you want without much trouble.
The people ordering now are people who will upgrade regardless. Once they're released there will be another wave of people who needed to see them before they upgrade. Then there will be more who wouldn't have planned an upgrade, but after seeing them will want to.
When you say "everyone", do you mean from what you read here on a very small sample of the world?
They won't actually sell out of phones until each store doesn't have any at the end of the launch day. I guess that would be a fair umbrella to cover what "sold out" means?
Yes there is a shortage. No one has got one yet. They took my money. I have no phone. So there is a shortage. And exactly how do you preorder? Either you place an order, or you don't.