If by Faulhaber motor you mean a brushless non-iron rotor type, then you should really prefer the 60905, which controls the motor with much higher frequencies than the 60902/60901. 60901 is designed for (heavy) DCM DC motors with iron rotor and works in the low kHz range. (Some decoders for coil stator motors even work with <100 Hz !) Faulhaber needs >>15 kHz.

No, because the PWM is an on/off control of the voltage, it will only introduce a voltage drop and the motor will run only one way (well that is what you want anyway), but the pulse rate will still reach the motor. The problem is that the low pulse rate used for iron core motors (as used in the Marklin 'universal' AC motors) will shake the windings in the motor to a point where they will fall apart as they have insufficient mass to act as a filter for the pulse rate.

If you could drive the motor from a function output then you won't have the problem. I suspect a headlamp output may have enough current capability to drive the faulhaber as these take a relatively low current, and the headlamp outputs are capable of driving an incandescent lamp. Would be worth checking the motor draw to check if this is possible.

My old dealer/repair tech. installed a 60901/60902 decoder in my #3511 Lady of Wurtemberg C. I have the manual for the decoder.
It buzzes at slower speed, but runs well. Should he have used a different decoder? It was done back in the late 90's.

Its the "wrong" decoder, even if it works the motor will probably not last as long as it might do with a correct decoder. It's more information on the stummiforum about this, but you have to search.
So depending on how much you have used it, a new mld3/msd3 or similar is recommended. And remember to select the correct motor type and do the automatic calibration. This will likely make it run better also, unless the motor is too far gone.
Depending on how the upgrade was done, a new decoder board might be useful. eg something like this: https://luessi.ch/eshop/oc/index.php?route=product/product&product_id=2438&search=Class+C (note the clip for the centre "pole", so you need to check your locomotive in order to get the right parts).

My error, I was in confusion between brushless and coreless. Brushless, that's Sinus...

English is not my best known foreign language, I am better in german, french and rhine-franconian.

I didn't have a decoder suitable lying about so I've just opted to order the 9301.7 with the decoder built in, for the approx 27Euro extra i feel i have taken the lazy option :)

The wheels from the trains leave some dust on the strips, so it seems as if it helps. There are several performance reviews available if you google it.

Well, I don’t have a 12 V DC source, only several DC sources at approx. 20 V, so I would have to arrange for a potentiometer or similar anyway. Also, I want the flexibility to be able to use the stubtrack for staging trains also, i.e. not only for cleaning. This is easier if the facility is controlled by the PC software. Finally, the decoder provides load regulation, i.e. the wheel cleaner doesn’t slow down that much, when entered by heavy engines vs. light cars. It’s a huge difference now compared to when i used a function output of a decoder for powering the wheel cleaner (as described in a previous post).

In post #14 i declared that the decoder is installed and everything fine. In #18 I stated why I wanted a decoder to control the wheel cleaner. I.e. no need for a separate DC charger, at least not for this animation.