0) I don't think you need to get into the details. I think it's rather obvious you are attempting to recoup something via the legal system by the way you have worded your questions.

1) IANAL, but most likely no, there is probably not a "legal definition" of what a full fare is, but I would grant that there is most likely a common conception of that a "full fare" is.

2) No, I wouldn't agree at all. Why would you expect that a full-fare ticket in one direction would be required to be the same price in the other direction? Although that seems lovely in order to comply with the laws of symmetry, I see no reason why it would apply in reality.

3) No, prices could be set on any number of factors, including load, demand, equipment type, fuel costs due to expected headwinds, etc.

I think I see where you're going with this, but I don't think you're ending at a correct conclusion.