I disagree with your notion that there can be options other than white and nonwhite. People may disagree on exact definitions of each, but I don't think there is a third state. What would that be? Nonwhite encompasses all options that aren't explicitly considered white. If there is a third state, I'd agree the meaning becomes ambiguous, precisely because nonwhite ceases to logically negate white. But I don't see a third state.

The following assumes there is no third state between the two.

If a person is making a statement like "I am not nonwhite," presumably it's unambiguous what they intend, yes? You may disagree with their definition (to use your example, maybe they are 80% white and define that as nonwhite but you disagree), but their intended meaning "I am white" is not actually ambiguous. Would you agree with that?

The alternative proposal for that last sentence is essentially that it's a weaker statement - rather than implying that one could probably indict, it implies that it's "not improbable." So in other words, if "probably" means at least an 80% chance, I'm claiming the sentence says there's at least an 80% chance of conviction. The other person is interpreting it to say that there's at least a 20% chance (not improbable), a much weaker statement.