By monitoring the pressure of the high and low sides and observing other physical characteristics, your mechanic should be able to tell where the problem is. If the system was working last year and not working now, the chance of having a leaking system is very high.
What tests did he do telling him that you had a bad compressor and a clogged "pipe"? The 300D has other control devices also controlling the temperature. They need to be tested too in a logical manner.