The fuel issue sounds like the siphon pump o-ring failure. The siphon pump transfers fuel from one side of the saddle tank to the other side and it contains an o-ring that can get displaced. Once this o-ring is displaced, the siphon no longer works, so the car gets starved of fuel at about a 1/4 tank.
Member
andrewwynn wrote a very thorough post on this issue: -
https://xoutpost.com/bmw-sav-forums/...stic-help.html
For the electrical issue, I agree that the first thing to check is the alternator - unlocking the OBC menu so you can view the alternator/battery voltage while driving is a great place to start.