Hello, I hope the following assists you.
I would be very suspicious of any engine damage after an oil leak, depending on how much oil was lost the engine could have suffered a loss of oil pressure which could cause serious engine damage. The oil pressure light goes out at around 4 PSI but this pressure isn't enough to protect the engine so although the oil light didn't illuminate the pressure could have dropped enough to cause damage.
You need to find out exactly what the garage says is wrong with the engine, transmission and turbo. It seems very odd there should be a fault with the engine and transmission. The turbo can be damaged by low oil pressure.
Having to take the car back several times for a leak would in my opinion put some blame on the garage, find out if the sump was actually replaced, it is also possible the sump was distorted enough to block the oil pick up strainer which again would cause a lack of oil pressure.
As I said earlier find out exactly what the garage says has failed and why and go from there.
I hope this helps but if you have more questions regarding this let me know and I'll reply as soon as I'm able to.