Glad you got it fixed. I do not understand why this took so long to diagnose. There should be a schrader valve on the fuel rail. I would have tested fuel pressure with the vehicle off, then I would have gotten the vehicle running and tested fuel pressure. I do not know the BMW spec; but, I would expect it would be somewhere in the 45-55 lbs range. From there just hit the throttle, see fuel pressure rise 5 or so lbs (this tests pump performance). Pull the vacuum line, again we should see fuel pressure rise 5 lbs or so (this will test the regulator). Turn off the vehicle and wait 30 minutes or so, see how much pressure bled down. It should be nearly no change. Leave it overnight and check the pressure again. If the pressure bled down, you are losing fuel pressure.

A leaky injector will make for a hard start as well as the car running rich on one or more cylinders at start up. You may see black smoke. A bad fuel pressure regulator will allow the fuel to drain back into the tank, so you will not have the rich condition at start up. A quick check of the exhaust at start up and pulling the plugs will tell you.

Sorry for the long reply, it just bothers me that modern techs struggle with the basics. This is something that someone with basic mechanic skills should be able to diagnose.