I didn't read this whole thread. My house is less than 10 years old and we moved in 5 years ago. It has a double car garage which does not have an entrance to the house from the garage. 2 walls and the ceiling are part of the house. I don't normally let my cars idle in the garage for more than a minute. I don't leave any liquid containers open for very long. My house never smells of gas or exhaust fumes. What does smell is my clothes when I am working on a car. If I don't put on fresh pants and a shirt within say 3 days my bedroom will smell like a shop. I usually put a fresh shirt on each day but sometimes I will wear the same pants for a week. I do put clean clothes on later in the day when I have finished working in the garage.
The strongest smells I find are cleaning solvents and spray paint. Lacquer thinner, grease & wax remover, used oil and spray primer are the worst. Rags that have any of those liquids on them are stored in a sealed plastic bag and then thrown out. I don't normally spill anything on myself. It seems that these smells just get into the air and then into my clothes. They are more noticeable in the winter when the large door is closed compared to the summer when the door is open and I have 2 large fans blowing from the back towards the front. It may also help if you rig up some kind of sensor to turn those fans on for say 2 minutes when you enter and leave with a car.
Crushed rose petals soaked in lavender poured down the carb will sweeten the exhaust fumes.
Don't tell my wife that I buy roses for the car and not her. lol.