availablity simply means when the unit is offered, sometimes a unit is offered many times in a year, for example, ACST101, it is offered in D1,D2,E1, so you have to choose a session to fit in your timetable.
I agree with u that it won't be better off, but i still jave different value of DWl.
the difference between the loss of total surplus and tax revenue is the DWL.
i think the loss in cosumer surplus is 0.5*14*(50/3-12)-0.5*17*(50/3-11)=-15.5
the loss in producer surplus is...