The D-types just divides the 555 output by two generating a square wave output from both.
The issue with that though is there is nothing in the circuit to ensure the two D-Types do not start out 180 degrees out of phase.
As you suspect the pot adjusts the frequency.
The D-flipflops are connected to provide a divide-by 2. I assume the main reason for those is so that the output duty cycle remains 50/50 even when the frequency varies (Which is not what comes out of the 555.)
The FET gives and inverted output but they could have gotten that by using the Q-bar output so maybe they needed a specific drive characteristic.