any hole in your exhaust system will cause the car to run a bit differently as it affects the breathing of the engine. If the flow of exhaust gases through the exhaust has changed due to the holes it could cause the lamda/o2 sensors to not receive a true/accurate reading of how the engine is running - that could possibly explain the reduced MPG (if the engine thinks the air/fuel mix is incorrect it could be trying to compensate by using more fuel)
If the converter is partially clogged it can cause the engine to overheat which can cause serious problems. Other than that if will not harm the engine.
Yes
No, it will not cause the engine to knock.
yes, if the Cadillac converter is not working properly it can cause you to loose power and may in the future cause you to loose your engine.
clogged up catylatic converter.
A failed catalytic converter is one of hundreds of parts that can cause the check engine light to be on.
Yes, an engine misfire will cause this and will ruin the converter. It may also be partially clogged.
A bad coil can damage a converter. If the engine is misfiring, it will cause raw fuel to get into the converter and that can cause failure.
A bad catalytic converter can cause hard starts if it has become clogged. This produces excessive back pressure in the engine.
That depends on the engine and the fuel you've used. If the engine starts burning oil a catalytic converter can fail rapidly. Some fuel additives can also cause a catalytic converter to fail prematurely.
Yes, it is possible. It will also cause severe loss of power.
Yes it can.