at least I can sched some light on the question of two antenna connections.
There is a main and an aux antenna. These are used for antenna diversity:
What it basically does is it avoids an effect called fast fading. Due to reflections of the radio signal and the short wavelength even millimeters can drastically change the receive energy of a radio signal. Using two antennas the radio chip listens on both antennas and chooses the strongest of both signals - very roughly explained.
2.4GHz Wifi and 5GHz Wifi are even multiples of the frequency which means that the same antenna can and will be used for both frequencies.
Finally we have Bluetooth in the Librems too (currently not supported in PureOS because of non-free firmware). Bluetooth is also using 2.4GHz. The Bluetooth chip is on the same card as on the WiFi, actually it is a companion of the WiFi chip. Since there are now two radios using 2.4GHz they must coordinate somehow. This is done by so called Bluetooth-Wifi coexistence, not by a second antenna. The two chips have signals to notify each other about who is going to use the antenna.
So bottomline: Two antenna connectors for diversity, shared for Wifi (2.4GHz and 5GHz) and for Bluetooth.
Since the antennas are placed into the LCD assembly and the back side is aluminum they are shielded off in the backside. Turning the laptop around a little can drastically improve the reception quality.