Seems to me you have two separate issues here:
- The corrosion issue
- The undertray.
Did you get a written quote setting out the scope of the work to be undertaken and if so was removal and treatment of the corrosion included? If you did and the work was not completed as per quote, then you have a legitimate complaint and should pursue it with the garage. If you didn’t then ‘Caveat Emptor’ is the advice.
As to the undertray that’s a slam dunk. If the tray had to be removed to carry out the work, then it should have been put back in original condition, at least.
Sadly it is common knowledge in the motor trade that customers often expect more than they asked and paid for. Some years age a customer booked their car in for a minor service. This was done as per manufacturers schedule. 1800 miles later they returned to complain that we hadn’t done the service correctly as the alternator had just failed and why hadn’t we dealt with this when we did the service!!