One of my concerns would be the gas that was left behind in the delivery system between the fuel filter and the injectors and the return lines and relief valves.
The detergent qualitys in gas that meets the BMW test will eventually clean things up but that takes time and never seem to fix a stuxk or failed item.
As far as the injectors, When you pull them to replace the seals is when I would take them to my local Benz/Volvo/BMW/SAAB dealer. they will all have bench testers that will check the spray pattern, leak down and opening pressures, etc. Injectors for CIS runabout $30 each so you'll have to compare the $45-60 they'll charge for the 30 minutes it will take (although they tend to charge a minimum of an hour) to the cost of 6 new injectors. There's always the possiblity that some ofthe problem my originate in the dry injector seals (seen that one) and stuck or sticky injectors.
Don't give up.