Regarding the negation, you can consider it in a few different ways, depending on your use case.
If your qubit is "all values", the negation is the lack of any value, e.g. "a measurement that results to 0".
However, in most cases, you will have a segment within the possibilities. E.g. one qubit can hold "vertically polarised light at frequencies between X and Y at phase P", then the negation could be one of "horizontally polarised light at that frequency and phase", or, more common, same polarization and frequency but the phase is 180 degrees off P. That way, if you add them together, you get the cancellation. Another negation is "all other possible combinations".
However I must note that when you call a qubit "all possible combinations" that's not a typical qubit value.
Think of it like this, "binary" is "0 or 1", but a "binary value" is only one of either 0 or 1. A qubit value is, depending on your interpretation, photons of some polarisation, frequency and phase, or electrons of a particular spin, or some other combination of those. Some of those can have multiple combinations, for example a qubit value could be the combination of a bunch of photons at different values. Or, an electron with an "unknown spin". Or an electron with an unknown spin that's the complement of another electron at another unknown spin, but when you measure either you will reveal the other, and so on. So, only some qubit values will have "all possible combinations" i.e. "when observed, it could literally be anything".
The internal of "what was the value before you observe it" is, well, controversial, and to most people, almost irrelevant, and even, "not good to consider".