Secondly, what is the torn brass/copper that appears in the "inside" view of the cap? Is it. possible that when the cap screwed down, it "seals over what used to be a nice hole?

I think that you should be able to gently "floss" out the corrosion with a guitar strings or wires.

I'd probably look at demounting coolant reservoir and taking it to a radiator shop. Then I'd have them ultrasonically clean it. I would not use acid given the way it looks to me. !