Graviton are hypothetical since they have not been observed yet. Actually, their properties are such that we cannot observe them with the experimental setups of today. However, theoretically, in the framework of quantized gravity (which is also an open problem), they are well-defined.
So in short, you are right with the graviton.
On the other hand, there is no such a thing as a negative mass. One could get 'effective masses' that would act as 'negative masses' in some context, but the word 'effective' is important.
For the rest of the question, I didn't get it :D