I believe the diagnostics are telling you there is no trigger for the #5 coil. Surely P0305 is telling you the cylinder is weak/no trigger, there is no question there. I do not know what the other code means because it is not in OBD II format, but from what you said about the code you have, the diagnostics are telling you that the trigger is not there. This tells me that the control unit (DME) has failed for some reason. The good news is, you don't have burnt valves or worn rings on the affected cylinder.
You are on the right track when you look at stuff that affects a single cylinder, and you stray from the right track when you start looking at cam sensors and fuel filters and such. These items should give a global problem, not a unique problem, when they fail.
I think the diagnostics are telling you _exactly_ what the problem is, and I think from all that you have said, it's the DME has no output on the pin that goes to 5. Do you know which pin you need to look at? I have a Bentley, and can look it up (I think). The final test you can do is to check that the continuity of the wire is not compromised -- you need both continuity from one end to the other, but you also need there to be no short to ground.