Been using Macs since system 7... owned Macs since OS9.
Step 5, 6, and 7 are completely unnecessary. You do not need to remove all those screws to replace the keyboard. You only need to remove the Hard Drive panel bolt in step 3, and the bolt in step 8 that has a keyboard logo next to it. You can confirm that this is the only bolt necessary to remove by viewing the underneath of the keyboard in step 10.
Your images show incorrect placement of the rubber spacer. Step 20 shows correct placement. It is just above the scan code sticker on the motherboard. That is where the ribbon cables roll over, so it makes sense.
I assume the thick side would be facing the rear of the phone, but can not be sure.?
Under. There is nothing to ground if it went over the pcb. Look on the backside.
Note that the actual keyboard is sandwiched between the top case and an aluminum backing panel that is plastic welded together. Because of this, the keyboard can not be replaced without modification to these plastic weld points.
I actually purchased a replacement keyboard only to find out that I should have purchased the whole top case unit.