It sounds like it is not corrosion, so it is something else. Yes, SD should be pretty low in all around humidity, and there were no spikes in iron, so less chance of corroded cylinder walls. Jug inspection through spark plug hole seems in order, but inspect the particles magnified, or send a collection of particles to Blackstone for metallurgical analysis. That would help identify the source.
Lifters could be another consideration, but an analysis should help narrow that down. Sorry, we could not be more help.