The dealership overfilled my 08 subaru legacy with oil, what can go wrong?

To be more precise they never took the old oil out of the car. they changed the oil filter, and added 5 more quarts. so my car had an extra gallon of oil. I started the car and smelled the oil. I just umed they dropped some oil on the engine and it needed to burn off. I made it five miles down the road and smoke came out of every part of the engine. I called the dealership and was told that it was fine to just drive. I took it back and demanded they check it. on the way back the car was sputtering, stalling, and smoking. they confirmed that they never took the old oil out of the car but are insisting it is fine. what do I need to worry about. I don't trust the dealership at all. they even wanted me to take it home saying they did not have time to fix it that day. I refused to take the car, and they are redoing the oil change I am just worried that they just killed the life of my engine. The engine is a boxer engine with only 10k on it.