If your existing OBC is a 13-button, no sweat. If not, I believe the 13-button has a different connector and harness than the 6-button that comes in the 318is. I could be wrong, but don't think I am.
Yes, the one foglight should come on. Unfortunately, finding a short can be a painstaking process, but at least you know what circuit it is on!
First thing to do is physically inspect the headlight wiring. Unfortunately, a fair chunk of it is behind the front valance - the passenger side light and foglight wiring all run between the radiator and the front valance. Hopefully you don't have a problem there, as it is truly a pain to get to. Look for obvious breaks, cuts, etc. Make sure the wire doesn't feel stiff/burned anywhere. Given that you already had a too-high-rated fuse in there, a melted harness is a possibility.

If that doesn't yield any success, break out the multimeter - remove the fuse on the circuit, probe the connections so you know which one is +12V and which one goes to the lights. Check for continuity between the not-12V-one and ground, then start wiggling the wires to see if you can make it go to ground. If you can, there is your short.
Not much fun to do.
I'd start with the passenger side, since the foglight connector is missing - that may be part of your problem right there.
