I get the feeling that you are attached to the 4s in some way. Nothing wrong with that, but I would say do what feels right. Sounds like getting it repaired and getting an SE is your ultimate desire; go for it!
If I were you I'd get an SE, sell the 4s, and worry about a backup if and when I needed it. FWIW I am the same way with phone insurance. I;d rather throw $10-ish aside monthly in case I need to use it in the future rather than drop $100 down up front for Apple care. I;ve saved enough money not buying Apple care over the years to buy a brand new top of the line iPhone. In short, I am already ahead.
As far as shipping, they have always done overnight on phone launches. I suppose worst case is a week, but i don't really expect that.
As far as the Apple store, it's tough to say. This isn't a flagship device. We don't have a good idea of stock (though I expect it would be plentiful, to be honest, especially if not picky about color). Apple stores inside malls also often don't allow waiting in line earlier than 6am. If you plan on getting in line I would ask what their plans are (often they will open two hours early on launch day too). Getting there a couple hours early, if you plan to get in line, is probably a good idea.
