That is correct, Apple sells this adhesive/gasket to its AASP and all Apple stores will have them. I'm sure you can find it from 3rd party part suppliers, but it isn't going to be guaranteed as you need an iPhone press to press the screen back on correctly so that the adhesive sticks. Also, doing any repair not performed by Apple will likely cause issues with waterproofing as well.

It may possibly, but I only know of this being used in the higher capacity models of the 6S? Maybe the 64 GB SE has it, but its not a guarantee.

Prox looks to be the same, but iFixit didn't reveal that part to us.

Power volume flex is a bit different, I believe that it has the NFC antenna on the end of it.

Im going to say that the extra bit on the power volume flex is the NFC antenna. That is the only reason for Apple to add that extra little bit. All of the other antennas look to be the same from the 5S so this would be the new antenna for NFC.

No phone has this many layers in the screen. The 5-6 had the glass, polarizer, then LCD, then the backlight. This has a lot more parts now, its not complicated if you know what goes where, but you need to assemble the pieces in the right order for the screen to be flawless.

Repair shops can replace just the glass if they have the right tools (what is shown above and more). At the Apple store, Apple will continue to replace the screens as a whole assembly, and send the broken screens back to where it is that Apple refurbishes them out (probably China). Apple will separate the screen and replace anything that it feels is necessary and then send it back to be used for another screen replacement.

iPhone 6 4.7 inch model teardown too? Or only Plus for now? Even thought it's the same internals on the board it's a different design after that.

No unfortunately you can not.. The camera is different. The iPhone 5S has the connection for the camera on the top of the board, and the iPhone 5 has it on the bottom of the board. So the camera connection is on the under side of the 5S and the 5 has it on the top. Also the 5 has the flash connected to it. The 5S does not.

Just different chips soldered in the same place on the board. The circuitry is the same for each chip allowing Apple to the same board just different chip.

No. You would need an iPhone 5S screen assembly.. The connections are in a different place and are different making it so you can not use an iPhone 5 screen on an iPhone 5S/C

The iPhone 5C and 5S LCD and Digitizer Assemblies have to be the same. They might have different prox flex's (camera, prox, speaker) but they are both compatible.