If the engine starts and then stops when the ignition key is sprung back to run position, it still sounds like ignition switch, even though you have changed this. Check on the back of the switch for a soldered dry joint or broken wire. other than that, it could also be a relay of some sort. Try the fuel pump relay. If there is a strong smell of fuel, then the fuel pump relay could be stuck in the on position thus the fuel pump is delivering too much fuel for the engine to handle.
Take the cap off the oil filler and have a sniff for petrol. If it is over fuelling, you may have fuel mixing with the oil as it runs past the pistons. This will give you a clue as to what is going on. Once the car is going, go for a drive but be carefull with the throttle. Fuel in the oil is ok and a run will make it evaporate away. But as I said, initially be carefull with the throttle as the oil viscocity will be reduced.
(Oil dillution with fuel is used in some aircraft piston engines in very cold climates to ease slow cranking problems caused by freezing cold oil.)