........ Oxygen is inseparable from oxidation (rusting).
yup
..... I think it's more likely that moisture was getting into those hidden areas and staying there longer.
yup, again
first of all the term "stainless steel" though popular & widely used is not the "correct" term for the material under discussion
the more correct terminology is "corrosion resistant"
btw all "stainless steels" will rust / corrode under the "right" conditions
another little know fact, so called "non-magnetic", 18-8 stainless steels (typically 300 series) can, when "worked" by certain processes, in fact, exhibit magnetic properties
cheers Bob