All silicon capacitors are relatively expensive, they're specialty parts that generally go into expensive things (mmWave airport scanners, optical backhaul, etc).

It may seem overly simplistic, but 2-port models for capacitors can be configured in series (as shown here) or in shunt (much more common for power distribution network applications). So, as basic as you might think this diagram is, it is entirely expected to be there so that we instantly know if the S-parameters are in series or shunt configuration - it would be more unusual if it were missing.

Same here. That's made my week.

I never understood why nF is less frequently used than uF or pF. To me, the logical thing is to express the capacity in units that are between 1 and 1000, like it's done with resistors and inductors.

Certainly not at these frequencies and corresponding high capacitance values; getting 22nF out of a PCB even with ECM would require quite a large surface area and at 220GHz you can't have that since your wavelength is so short; you'd just be making a giant resonator instead.

No, not if you want 22 nF. That's a pretty substantial capacitance. It would need to be quite big on a PCB. That would mean significant inductance, and thus, bad performance at high frequencies.