In the scenario I propose, 200 is less than the 400 requested, so it's incomplete. The cache is permitted to retain the smaller request, and return bytes that fall exclusively within, but like I said, I don't think it's free return 200 octets when 400 are requested. If it was why would it make the other statements?

I do think the cache is allowed to retain, and respond for the 200 bytes. I don't think it's free to ignore the header updates, nor do I think it's free to return half the requested bytes in lieu of extending the existing cache.