> The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
Yes. FM radio is "narrow band" which gives it additional noise rejection properties; however, it's measured against the total available signal content not merely the audio portion of the content.
So, your pilot wave and RBDS and any additional carriers, if present, reduce this facility.
> FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better
FM receivers often move and are often in highly reflective environments. FM is far better suited to this than plain PM.
The modulated signal that is transmitted on the air has a much higher bandwidth. How much higher may differ between various broadcasting standards, but it can be e.g. 10 times or 20 times higher.
The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
When the bandwidth available for transmission is limited, FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better (and optimum), so that is what is used for digital communications limited by noise.