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One at time:
Swap ignition coil.
Then spark plug.
Then injector.

If misfire doesn't follow, you have a problem limited to that cylinder, possibly with the opening and closing of the valves.
A misfire is detected using the ionic modules. That means, the quality of combustion is not as expected. Is the throttle sticking, working?

Do you have the BMW specific codes? or is the misfire read off from an OBD2 device ?
 

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SAP = Secondary Air Pump. SAP gets kicked off on cold starts only, to pump air into the exhaust headers, and burn the extra fuel that is injected into the cylinders and into the catalytic converters to heat them up quickly.

Is the fuel injector in #5 leaking by any chance? Can you take out the rail and crank the car and see how they spray ?
 

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I know my cats are bad.
Negative ! Cats do not go bad. The smell of gas means your Secondary Air pump may NOT be working, so you get misfire only on cold ! Tada !

Do this: early in the morning, start the car, you should see water vapor coming out the exhaust , thick white "smoke" that is purely water vapor. If you don't see this, there is a good chance your SAP is dead and not blowing air.
 
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