The back end of the microphone riibbon cable may be stuck down with a bit of glue under the flap. You can gently loosen it with a flat spudger.
It’s not clear what you mean by “This step is not needed.” If you want to remove the logic board from the upper case in order to put it onto your replacement upper case, you will have to remove this screw.
I found the same. If you already have speakers in your new upper case, you can leave them. When you put the logic board back in, it will be a tight fit. I had to start with the corner near the right hinge (the Thunderbolt port corner) and work it in to place.
Yes, make sure you have a good quality screwdriver that gets excellent purchase on these screws, and bear down to make sure you don’t strip them. I stripped one and had to drill it out with a 1/16” carbide drill bit. I’ll end up with only two screws on one side but I imagine it should be OK.
MBA 2017: At this point I had to deroute the antenna cable from a channel along the back edge of the the upper case. Good to make note of this routing.