May or may not be related or possibly answer part of your problem .
What can happen on some m54 engines if they don't fire first turn or are only driven a few feet and switched off again.
Fuel leaks into cylinders loosing compression making any subsequent starts very difficult.
Happened my self last year when I pulled the car up a few feet closer the house to avoid frosty weather, the following day it took a full minute of turning the car over before it fired, ran like a bag of shite for a minute but after that perfect and not a bother since.