In the first example,
No person could have been more nationalistic.
there is an implied subject of comparison, probably a particular person who has already been established as the subject of discussion. A more complete statement would be,
No person could have been more nationalistic than Joe.
The literal meaning is, "it is not possible for a person to be more nationalistic [than Joe]", or "Joe is as nationalistic as it is possible for a person to be."
Such phrases are, of course, often used with an element of rhetorical exaggeration or hyperbole, so it's possible the speaker doesn't actually believe the statement in its literal sense.
I couldn't have been better.
Here the implied comparison with other states of being you might have had at some moment. A more complete statement would be,
I couldn't have been better than I was [at that moment].
There's no "supposed person" here because what you are implicitly comparing is not between you and other people, but between how you were and how you could have been.
This can possibly be changed through context:
Joe was good at chess. I couldn't have been better (even if I practiced daily).
The sentence (A) isn't correct grammatically.
"I couldn't open the door" is in the past. As a matter of fact, you are locked in and unable to open the door at the present time. So the correct sentence should be:
I am locked in here and cannot open the door. Can you help me?
The second sentence (B), which is in the past, is correct grammatically. Could is the past of can.
Best Answer
couldn't have in the interogative implies that I think you could have.
could have does not indicate my opinion.
"Couldn't he arrive earlier?" means I think you could arrive earlier next time, and I am asking you if I am correct.
"Could he arrive earlier?" means I do not necessarily have an opinion on whether he will be able to arrive earlier.
This is true for in general for negative interrogatives. They imply my opinion, and are often used to disagree with someone.
"Shouldn't I talk to John?" Means I think I should talk to John, but I want your opinion.
"If I go to London, should I visit Buckingham Palace?" is neutral. I am not expressing my opinion.