2005 Chrysler 300c oil in coolant reservoir

So to start off, I drive my car 20 minutes away last week and when I leave to start it it shuts off so I get out and check the oil and the coolant. The oil cap looks fine and the oil is full and I checked the coolant reservoir and it looks like there is oil in the coolant so I take it to a shop and they say the water pump is leaking so they fix that and flush the coolant or flush the reservoir out and fill it up with coolant and send me on my way and then there’s a check it a day later and there is more oil in the coolant reservoir so I take it to somewhere else and get a diagnostics test and they did a chemical test and said it came back positive for and they think it’s a blown head gasket so I went back back to my mechanic, and he said he has never seen a blown head gasket cause oil in the coolant reservoir ever, and the car is not overheating. It does not misfire the only thing that made me check the coolant reservoir was it turned off once I started it a week ago, talked to more mechanics around and one said something about transmission lines run to the radiator and might split and got oil in the coolant that way. Any ideas !?