The immutability of the 1MB blocksize still stands if we throw out the word "protocol". Satoshi knew that once he slipped that blocksize limit into the core client, it would be there forever,
I think the interim limits of 250KB and 500KB invalidate your assertion.
I assume you're talking about the default, "soft block limits" that miners were free to choose, as long as they were under the 1MB hard (consensus) limit?
Yes. I'll read your references before further comment in this direction.