Even with ideal "perfect" construction, the diaphragm will deflect to one side or the other due to minute random air currents or acoustic energy impinging on it. Once it starts moving off center, it will continue to deflect until the Force from the negative compliance term is balanced by the mechanical tension term. The deflection will take on a curved shape with larger charge content in the center where the diaphragm is closest to the stator. To solve the exact shape and deflection magnitude requires solving nonlinear differential equations. I've seen a few JASA papers on the topic, but nothing recent. Rather than dive into heavy math, pick a reasonable average deflection of 10-20% and see what that gets you. Based on the Force equations, you would expect about 78dB of hum with mic/ear 10cm from diaphragm if there was 100Vrms ripple on the HV supply and no resistance in series.1) have you any idea of the magnitude of these imperfections, could this, for sake of easier calculation, be addressed to one defined area and it's dislocation distance.

If so, math could help us trying to calculate the effect of modulation by the ripple.

In general it is not an issue because of the low level of ripple on the bias supply, and the LP filter effect mentioned below. Usually when I have experienced it, it was due to leakage currents putting high level of ripple on the bias supply, and/or low resistance diaphragm coatings.

Yes. The hum will be reduced as expected based on response of the 1st order LP filter formed by all resistances(external and coating) and the associated capacitance. As MarcelvdG has described, this is very difficult to quantify by inspection since the diaphragm resistance is distributed and the deflection is not uniform over the surface.2) since our ESL's are working in constant charge mode, what is the time to change the charge because of the bias ripple. When this takes quite some time, the charge does not change with the same amount as the ripple's magnitude, therefore most likely diminishing the effect of modulation.