Hey Steve - just asking - are you sure BOTH fuel pumps are working properly? You probably have but I always find that on car problems that are perplexing - the cure is something that we end up saying "Ya gotta be kiddin". To follow through:
1) fuel pumps engaging and putting out proper pressure.
2) check valves good and holding pressure
3) accumulator holding proper pressure
4)regulator properly set and working
5) injectors are good and not leaking
6) cold start injector working
If all these are met - the car should start instantly. If you know 1-5 are for sure good - that leaves maybe #6
How did you check you cold injector? If the 5th injector does not spray enough fuel in - the car will be hard starting as you describe. Did you also check the circuitry to the 5th injector? Sensors - relays?? One crazy thought - if you pull the fifth injector and put it into a jar and have someone crank the engine and see what you get out while it cranks. If you haven't tried this - I bet you will get very little or nothing coming out of the 5th injector and the car will start as enough fuel comes in from the other injectors over a few seconds.
Just a thought - I sympathize with you because I am too am a DIYer and am the same way with my Benz - if it was designed to run right - it should - and I will continue to bombard the problem til I find it. As I said above - follow the check list - something HAS to be measuring wrong - or it would work.