That's a good point. Technically you're correct about the blocksize limit not being part of the protocol, of course.
Though if everyone is running a client with a different blocksize, the network will become a clusterfuck.
No. The miners -- or rather 51% of them -- would then determine the max block size. No clusterfuck. Longest chain rules.
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.