Outrageous Bosch Hydrodiesel(Vereeniging) repair bill for engine overhaul when fault for vehicle smoking was just turbo seals

I'm taking this time to warn any car enthusiast out there about the very poor service I received from Bosch hydrodiesel in Vereeniging owned by Navine. My BMW e90 335i was smoking so I took it in for repairs at Bosch hydrodiesel. I understood that a diagnosis had to begin somewhere so I agreed to a valvestem seal replacement, even though I shared my past experience with Navine about having replaced valvestem seals on a previous BMW I had before and how I didn't think it was valvestem seals because of the characteristics of the smoking. The cost was R7826. Anyway the valvestem seals didn't repair the smoking, at that point I had started doing research of my own in BMW chat forums and found alot of people had the same problem which they had resolved by replacing turbo seals. I shared the info then suggested to Navine that it could be the oil seals on the twin turbos so we should look into it as stripping the motor should be the last option. Bosch hydrodiesel assured me that they had looked into the turbos and that the turbos were ok. They suggested we strip the engine to further diagnose. Their diagnosis was that my vehicle had overheated in the past and shrunk one of the pistons. I failed to understand as I had never overheated my vehicle and because I knew the characteristic smoking of wornout rings and pistons to be a constant blue-ish smoke from startup, especially when revving or accelerating. That was not the case with my vehicle's smoking. I had friends drive my car and I drove behind it...The smoking would come then it would go, but it would puff alot of smoke when deccelerating or coming to a standstill just as other drivers on the BMW chat forums had also experienced. Bosch hydrodiesel was adamant it wasn't the turbos so we opened the engine and reconditioned it, replacing the piston their diagnosis said had shrunk and replacing all the rings. The cost came to R47 460 but my vehicle was still smoking the same way it did. It was at this point that Bosch hydrodiesel decided they should take out the turbos and diagnose them. they sent the turbos to a specialist for a strip and quote diagnosis which confirmed the oil seals were worn out. I had to pay a further R11 000 for the turbos and my vehicle's smoking was repaired. I disputed the matter with Bosch SA and after their so called investigation, their ruling was biased and I was given no compensation if any sort.