The phillips screw at the bottom centre in the image above is recessed quite deeply in the case. Some screwdrivers with replaceable bits don't reach in far enough to engage with the screw. It is also very difficult to relocate the screw when reassembling. I left this screw out, but nothing feels like it is loose.

Great guide, thanks. I was able to fix a problem with the scroll wheel where it had a strange vibration/bouncing action after spinning it.

Another helpful ifixit guide. I was able to reduce the scroll wheel friction after opening it up and giving it a clean and a touch of lubricant. It should be usable for a bit longer now. Thank you