NVMe M.2 drives do work with the iMac 5K 2015 and 2017 as well as the Macbook Air 13" 2015 and the Macbook Pro Retina 13" 2015 out of the box with the correct adaptor. It appears that only particular versions and brand of the adaptor works well. The 2013-2014 Macs need a modded firmware to work well. @danj has mentioned in another thread that adaptors might have massive CRC issues, but he did the test 1 year back. No sure if the adaptors that the Macrumors users are using are newer versions that seem to work very well. Check out this link. Someone used a Samsung EVO 970 drive in his iMac 5K 2017 without any issues. The HDD is upgradable without a thermal sensor if you use a temperature control app. You can use Macs Fan control or SSD fan control. Both will work. The SSD only version comes with a 3.5" Hard Drive bracket but not the SATA cable.
The newer black Sintech adaptor sold on Amazon seemed to be working very well with a number of users on the Macrumors thread at Upgrading 2013/2014 Macbook Pro SSD to M.2 NVMe The thread title is misleading. A lot of users are using Samsung NVMe M.2 drives with 2015 Macs without issues. 2013-2014 Macs would require a modded EFI firmware or some workaround to work reliably.
The touch bar can be purchased separately without the top case as a brand new unit. Comes with adhesive on the bottom and a detachable flex cable. The touch bar connects to two ports of the logic board. One outputs power and display to the LED, the other receives input from the touch bar. I am able to get the parts if anyone needs it.
As shown here, the cables mentioned by @danj cannot be swapped out. I have pried open the clutch cover which is glued to the LCD bottom panel to investigate. These are the components, labelled, with clear explanations of what they do and how they are connected. This should answer anyone's queries about whether the LCD cable can be changed. The part that can be changed is not the answer. The parts that might be the failing ones, cannot be changed. Conclusion: 1. Apple needs to initiate a recall. 2. LCD display change is the only solution for now.
@reecee I have successfully removed the keyboard and installed in a new one. You need to get hold of a diagonal cutter that looks something like THIS Use it to grip (not cut) the rivet, twist it a little, and then pull it out with a pivoting action. You need to keep the rivets to reuse them. Hopefully you kept the some that were already pulled out when you punched the keys out from their socket. To install, clean the base with isopropyl alcohol just in case the keys get stuck to the frame after installing. Lay the new keyboard back and screw back the screws (remember their positions before unscrewing) Then use the flat back of a screwdriver, or the tip of a big enough torx screwdriver to punch the old rivets back. It can be done be is tedious. The most of the rivets will go in, but some may be too deformed to use.