Yes.
I can't find any evidence of this, but there's no reason to think it wouldn't be true. As for 1), If they're going to go to the trouble to make custom-form flash drives, you can be sure they're going to make as few different designs as possible to save money.
As for 2 and 3, You could try emailing [http://www.photofast.tw/products/GM2_SFV1_Air.html|these guys] and asking them if their aftermarket 256GB drive will work in a 13" MBA; the fact that they can fit 256GB of flash into an 11" MBA is a pretty good sign that it will work (electrically and driver-wise). It seems like their product is proof of 4).