To answer the first question, no, that would not be a correct assumption. US would prefer to sell all seats in full Y. Therefore, chances are that the entire capacity in economy class can be purchased as a Y fare. A portion of these seats then overlap in the other fare classes. If US' revenue management, for instance, thinks that there is a good chance that they can only sell the fare associated with T, then there could theoretically only be 7 seats left for sale in total (the same 7 seats in all classes). I have seen instances where US opens up all fare classes on the day or two before departure, after most had been zeroed out for quite a while. Probably would mean that they really want to sell a few extra seats, and don't expect anyone paying full Y to come along. Turns out that the flight was oversold, so just because I saw each class showing as a 2 doesn't mean there were 2x10 seats available.

For the second question, it depends whether US will oversell seats on that route. If there is only 1 seat left chances are it is overbooked in F.