Catastrophic gearbox failure

I sent my Forrester 2.5XT into the dealer for its 100 000 km service which from the service schedule in the owners manual should have included changing all oil. In total I was charged R17 000 for this service. After getting the car back, I drove only 400km before I had a catastrophic gearbox failure!!!! That cost me an extra R30 000 and that’s for a SECOND HAND gearbox which had already done 150 000 km!

My question is this: 1) How on earth was the imminent failure not spotted at the service, since there is a ring magnet in the sump to collect chips which was full of chips. 2)How does one get a catastrophic gearbox failure in an automatic when it’s impossible to crash gears? 3) How do I k ow that the oil was even changed, or indeed replaced after draining (the most likely cause of the failure)