1.- The size of the shaft relative to the pins and its position relative to the prop and boat do not affect whether or not, or to what extent, electrolytic corrosion will occur.
Pins are small and they are between prop and water...
so since stainless is less noble than bronze, pins will act as sacrificial anodes and may dissolve relatively quickly.
The first part of this sentence (besides being completely irrelevant) has absolutely no relation to the second part.
3.- The pins are made of monel because monel is galvanically closer to bronze than stainless is (as you correctly pointed out). However, this is to prevent galvanic corrosion, not electrolytic corrosion.