Carefully note the orientation of the hard drive cover before you remove it. I was putting in an SSD with no label and no clearly identifiable "bottom" so I made note of the SATA connector position relative to the cover.
These connectors are fairly easy to unplug. You can pull up on the wires GENTLY to remove them. I used a wooden spudger to pry gently and the connectors popped right out. Putting them back in is easy just make sure they are right side up and they snap right back in.
I had to watch a youtube video to see how this works. Then I realized you are just using the tools to pull the main board out toward the back of the mini.
This is a screw into the head of another screw below it that holds the main board. When I took out the fan, the top screw AND the longer 26 mm screw came out together so when I was putting my mini back together I thought for a moment I had lost a screw.