Don't know on this car but on Vw gas jobs we pull the fifth (cold start) injector out of the manifold and crank the engine with the high voltage lead to the distributor removed with the engine warm and aim the injector into a glass jar and watch for leaking. With the electronics on this car I would be hesitant to do that as I have learned from experience that the spark generated can reach 1 1/2" in length (Mercedes always seems to overdo it!)So I would agree with plugging the manifold hole and seeing if the injector is leaking with the engine warm. If the injector was not supplying fuel - the car would have starting problems -which is not the case - so a cold check is not required.
Was you car sitting for a few years in the past?
I would go with your 1-3 ideas first and put in stong injector cleaner in your tank or that $20 stuff in a metal can by Duralube or others called fuel system cleaner and conditioner - That stuff is what I used when I got about 100 miles with no missing - now that I remember (Bear with me at times - as I get older I forget somethings until my memory gets tweaked!) and drive for a while. If there is any dirt in your fuel distributor - it will also clean it as well as your injectors. I have learned a long time ago -sometimes the solution is very simple - but you get too complicated and miss the simple things.
Check this site for what the fuel pressure is coming from the accumulator and ask the guy at the counter if the filter will work at that pressure.
I just finished putting in a rebuilt engine and tranny in my son's car and will be finishing it off this weekend - bet I know where you will be!!!