So much for the "concentrate on the single cylinder" theory. It just goes to show, you can not make any assumptions with cars, and with e46's in particular. When I replaced my fuel filter last month, I had no idea that it would fix the idle issues I have had since I bought the car, but it did! I know people who have replaced every part imaginable to fix their idle issues, with no luck. And I go and fix mine without even trying.
As for your car, I would guess that the computer has a certain threshold for the number of misfires it will ignore, and that threshold is 5. It then reports a misfire in cylinder 5.
You are not the first person that has mentioned a cylinder 5 misfire problem. As I said in one of my earlier posts, I was trying to help a guy several months ago with the same error. I went back and looked, and he had not tried changing his DISA. He has dropped off from the forum, but I sent him a PM, and told him about your solution.
I'm glad you got it resolved, without spending too much money.