The drive I used is a 2.5” SSD that is 6.8 mm thick.
I'm not sure about the official part numbers, but I was able to find listings on ebay for the drive bars and cables when googling for "15-ax280nd drive bars" and "15-ax280nd drive connector".
I believe that the chips you see under the heat pipe (there's one at the top just after the turn, and then one at the end) are the CPU and GPU, though I'm not sure which is which. I can't imagine them not putting a heat pipe on the GPU.
Yeah, removing the back cover feels like you're breaking something. I found that slightly twisting it upwards (as if it were connected to the screen hinge) helps with getting it loose without it feeling too wonky.
Those two top T6 screws are too deep for my screwdriver to reach, I have the 64-bit set from ifixit. Kind of a shame..