Qantas provided exactly what you were entitled to and to call them the worst airline based on this seems bizarre
Given that you were booked in Premium Economy, you had no lounge access rights so they were quite correct in not permitting access. If U class had become available between booking and date of departure, as long as AS award rules permit, you could have rebooked in business. There was no way that they would just upgrade you at the airport
For the inbound journey, you will indeed be able to get lounge access since you are booked in business class. Although you may have paid AS the cost of a business award, they only booked PE with QF
Dave