Hi @rhoran 
This definitely sounds like an interesting workflow!
For the earlier stages (where an item needs to be created in only one board when the status is complete), I would recommend using the following automation:
When it comes to boards C & D creating an item in board E when both items are completed, this would be best achieved by having the automation set up on one board and using mirror columns to reflect the status of one of the boards on to the other.
You might like to use this automation:
That said, automations will always require a trigger, which in this case will be the changing of one status column. You would need to set this to be either C or D.
If you set the automation to trigger on the status change of board D and this occurs before the status of board C has changed, this will not create the new item on board E.
I hope this helps! Remember to mark as solved if this answers your question 
Best,
Dani