Doesnt sound good. I know the mods are keen for us not to be canning the dealer but I would not behappy with the result. You are very patient to accept that the engine fault was returned not fixed and the car not properly put together aftwards. I would have not have been sympathetic, however you do need to keep the dealer onside to get the car fixed. Is there any other dealer you can take it to? Does anyone here know of great service center? I am not criticizing the dealer, as no doubt they are trying hard. However I would not accept that after working on your car they could not identify the fault and gave it back to you with problems, and when you call they still have no idea. It may be that another dealer has a more experienced mechanic who can help. After all, not every dealer has the same mechanics, and even doctors send you to specialists.
Unburnt fuel smell is a problem. I believe there are sensors in the exhaust that detect unburnt fuel to adjust the mixture from lean to rich. Maybe one of the sensors might be misreading, but it could be anything. Pinging is also bad. With all the tech involved in these engines, i would have a look at the sensors as the pinging could be pre-ignition, causing incomplete combustion resulting in unburnt fuel and a lack of power. These issues can be hard to track unless you start replacing sensors.
Can they connect up a data logger to the ecu and go for a drive? That would be my request. If you can get the car to repeat the fault while the data logger is attached then they have something to work with. I am guessing that they didn't see the fault themselves when they gave you the car back. I believe that only some dealerships have these data loggers, so they may need to get one in and send you home with it for a days drive or so. It can't hurt to ask, particularly if they are starting to say that they are stumped.
Is there a chance it could be a batch of dodgy fuel? We had some in Melbourne a few weeks back. I would try to fill up with the best fuel you can get, not from your usual source, ideally the best (ie most expensive) BP fuel you can find, I forget the brand name. If the car runs better after that then that may help diagnose the problem, otherwise you can only leave it to the dealer. Less pinging on high octane fuel would suggest pre-ignition.
At least Gerhard doesn't think there will be a loss of value in the car. You can always sell it to him if things don't work out to your satisfaction:)