Coolant in crankcase

My daughters car was running “rough” so I took it to a local Firestone shop. The vehicle had not been running hot nor was the temperature gauge fluctuating. The shop replace all six spark plus, throttle control mechanism, a coil, and rear wiper blade. Picked the vehicle up late Friday before closing. After driving the temperature gauge began making big swings from cool to almost running hot. Checked the radiator and added a gallon of antifreeze. Engine was and still runs basically smooth; no real issues with acceleration or idle speeds. Next day checked radiator fluid level because gauge continues to fluctuate but never ran “hot” but close. Normal highway speeds gauge is pretty much centered. Radiator was low on fluid, so I added another gallon of antifreeze (wonder where the heck the fluid was going as there’s no leak). That is when I checked the oil dip stick and it showed over the full line by about an inch and oil was discolored. The vehicle is still running okay and idles fine. As of now my daughter is not driving it. Open to suggestions.