Hey, I’m not an expert either but I can give some clarification concerning what you labeled “special points for flashing?”:
They are what Kees referred to when he talked about using a serial connection. You can solder Rx and Tx to two of these connectors and attach a serial to it. That way you can connect a serial console to the phone without relying on screen for input and output. Unfortunately I have never done this myself and do not know what points to connect to, nor what baudrate to use, etc.
Concerning the expansion port, I believe you are aware of FairPhone/Electronics at master · dirkvl/FairPhone · GitHub
Have fun hacking!