What is the mechanism by which myelination reduces the capacitance of the axon membrane?
Circuit analogies don't 100% apply to myelin because membranes have complex electrical properties, but both of those explanations work and they are in fact essentially interchangeable: Take a membrane with distance d across the membrane and capacitance c. Then we add some myelin to get a new capacitance C at a new distance D. 

If you 4X the distance between plates (D = d * 4), C=c/4 (from the formula you posted as (2) ); if you add 3 extra plates (so now you have a total of 4 plates), C=1/(1/c + 1/c + 1/c + 1/c)=c/4.

Importantly, myelin also increases the membrane resistance, and because myelin is typically very thick compared to a normal membrane (~10nm for one layer vs. 500-2500nm for myelin), you can almost consider myelination to increase resistance to infinity (compared to the axial resistance of cytoplasm) and the capacitance to zero.

See this page for some more info.

Note that the reason these explanations are interchangeable is that there is effectively no distance between the added plates in series and no difference in capacitance for each individual capacitor/piece of membrane (for example, see this page).