Apple have enough market power to get the chips they want from Intel, whether or not they have been released to the market. Apple have shipped unreleased (to the public) chips before. Yes, it means that we will not see Skylake MBPs until early-2016.
The extra cost of the discrete GPU is not justifiable for most power users, so Apple will eventually drop it. That will bite for the few users who need maximum GPU performance at any cost, but will be great for the majority of power users who will be well served by Iris Pro 6200 graphics and save a few hundred dollars.
Yes, the banks are critical. Also more important than signing up retailers is expanding the customer base by ensuring that all new iPhones directly support Apple Pay (without assistance from an Apple Watch). I expect that by the end of 2015 Apple will not be selling any iPhone models that don't support Apple Pay.
I would not assume that every iPhone user who upgraded to the iPhone 6 or 6 plus upgraded from an iPhone 5S after only one year. My guess is that many of them upgraded from the iPhone 4, 4S, and 5 after two or three years. The iPhone 6 is not too big for some people's hands. It is too big for others. That's one reason why the iPhone 5S and 5C are still selling.
I think a 4" iPhone 6C is plausible. If Apple were to build it, I would be shocked if it were to not include Touch ID. I would be surprised if were to not include NFC, because it would be stupid to not include support for Apple Pay. That makes the A8 chip the logical choice, even if the camera and flash might be taken from the iPhone 5S.