Could silent aspiration during general anaesthesia cause bronchopneumonia and mortality?

Maybe, but. Probably not. An undetected aspiration is unlikely to have serious pulmonary consequences and even less likely to cause death. If you are experiencing this complication, you will have more data with which to base a judgement, like x-rays and blood gases. Discuss these findings with your doctor.
Yes. Aspiration, silent or not, can lead to a pneumonitis (pneumonia) which is an inflammation of the lungs w/o a viral or bacterial as a cause. This will increase morbidity for sure and possibly mortality if not treated promptly. Also not all aspirations can lead to a serious problem. Amount and type of aspirate is impt.
Unlikely. Intubated patients do have the possibility of passive aspiration around the endotracheal tube when they are in the supine position. This is usually the case in patients who are intubated for a long period of time, not in those undergoing surgery. The possibility of aspiration is why no patient is allowed to eat for 6 hours before an operation.