Amar.
You posted the same query in your own topic eight months ago, and failed to respond to the suggestions there. If you can't be bothered to look at it why should anybody answer you now?
https://www.audiownersclub.com/forums/topic/26340-audi-a3-16tdi-2011-randomly-cuts-out/#comment-100863
Shaf.
Whereabouts in the UK are you, and why are you suggesting parts from an American supplier? Are you aware that USA spec cars are different to UK and European spec cars?
Yes and no, it depends on the competance of the persons involved not the training organisation. I suggest you look for independent VAG specialists to get the best of both worlds i.e, a good knowledge of Audi cars without the corporate greed and indifference of the Audi company.
Yes of course the battery should be disconnected before cutting wires.
Are you intending to change the rear lights to a different type, such as tungsten to LED for example?
In your position I would demand, in writing, a replacement car on the grounds that the car is defective and the dealer has thrice proved himself incapable of fixing it.
It could be a long battle so keep records of all communications, which should preferably be written not verbal.
More likely he flushed out some sediment or removed some air.
I suggest you reverse flush the matrix alone, i.e. not the whole cooling system, with mains pressure water. You may be lucky to dislodge sediment and limescale enough to get the heating working again.
There's your answer: 8 quarts in the UK is 9.1 litres, not 7.6 litres, so you have added 1.5 litres too much. An odd thing to do because oil is sold here in litres, not quarts or pints or gallons, so you must have done some conversion when buying it.