Yes, I know about what happens when a swap takes place, but the cowling came from RR in the first place, so RR is liable if they supplied a faulty product.
I very much doubt that is the case. In general terms, the engine nacelle is designed by the airframe manufacturer, in this instance, Airbus. Airbus determines the shape of the exterior, and the engine manufacturer (Rolls Royce) has significant input into the interior aerodynamic flowpath design starting with the entrance lip and perhaps input into the thrust reverser requirements. Once the nacelle design is firmed up, it is turned over to a nacelle manufacturer. It is Safran for the Rolls Royce Trent engine. The nacelles are supplied to Airbus who is the customer.
Airbus and RR would both be liable if the cowling is faulty.
Not true, if anything, responsibility would be between Airbus and Safran depending on the fault, design or manufacturing.