Does this Thinkpad really have only one RAM slot? Or is there another one on the back of the board (i.e. under the keyboard)?

Here is a more detailed view of all the cables that have to be unplugged before the motherboard can be extracted: https://ibb.co/jcgwUQ

Also, be aware that, even after all screws and cables are removed, it will still require a little bit of force to remove the motherboard, due to the thermal adhesive on the CPU and GPU. Do not be afraid to pull the motherboard! As long as you followed the instructions and disconnected all the EIGHT cables and SEVEN screws, you are not going to damage anything, it’s just thermal tape!