what is the reasoning
 

for changing the oil during the winterization process instead of at dewinterization?

You should allways change the oil in the fall so that the contaminents dont sit in the engine all winter.

contaminents? will these hurt/pit the cylinder wall,pistions or settle to the bottom of the oil pan?

The reason is twofold. First, you want to eliminate as many as possible suspended particles and contaniments. If left in there, it will be sludge by springtime. Second, used engine oil becomes very acidic after it contains combustion byproducts. This acidic oil will contribute to corrosion.

I will change my oil before winterization substituing 1 quart of marvel mystery oil. Will change oil again in the spring. Seems to help. When I disassemble the engine for routine maintenance, everything looked as good as the day it was assembled.