There are quite a few complicated problems in the car. But I don't want to spend too much on an old car. Maybe you have a special interest in it  we become what we behold

Was the car running okay before this happened?

P3091 points to low fuel pressure, but you've already addressed the items I would have suspected. It is quite possible this could be a DME / software issue...

hello all new member here. this is my situation 2010 MCS stick shift.:= got the half engine light and code P3091, and some misfire codes. car hard to start but will start and idles ruff and back fires. and will not rev out past 4k did some research and it pointed to HPFP. replaced it and no change. more research says intank fuel pump LPFP, replaced it and no change. so i replaced the fuel filter and the fuel pressure sending unit and no change. Then checked for batt. volt at fuel pump with key in on position -- no voltage. checked fuse its good. checked for the 3 to 5 second priming voltage with keyfob in -- no voltage. start the car and while the car is running there is voltage and the pump and vibrates showing it is running. So it must be a relay problem well BMW "F" us with a in circuitboard fuel pump relay. so i changed the JBE.-- no change. .checked the wire from the JBE to fuel pump its good. could it be 2 bad JBE (possible). did a bench test of the old JBE and the fuel pump relay is good. More research says it maybe a faulty DME but before i go down that rabbit hole i am hoping fo some suggestions. i already spent over $1K on parts luckily i am a DIY weekend warrior. i own the car for 3yrs its my daily drive and i have not stop spending money on this piece of shhhhh but its the most fun fun fun car ever. and i love/hate it and NO its not for sale