Philipp,
no, in Lesolees calculation (it's exactly the same I have done) that dR is NOT describing the series switch resistance.
This dR comes from the Hamon adjustment .. the series/parallel switch resistance is compensated by the adjusted leg of the three 120kOhm resistor chains.
That means, this leg is mathematically reduced by the switch resistances.
Either you simply assume that all three 120k (plus 2 * 10mOhm) are in parallel, or you elaboratley calculate the star/ triangle transformation, that might give 3 *10mOhm, maybe, but the Hamon adjustment condition says, that this paralleled circuit has to match exactly the 40k of the lower leg.
This Hamon adjustment condition you always miss in your calculations!
Therefore there is no black magic behind, but simple math.
Btw.: there is also no difference between the described switches in the assembly, because all switches / wipers are sitting on the same switch and are made from the same material.
The schematics are also available.
Frank