further correction - his father in law lived in a part (wing) of the Neue Residenz, it did not belong to him.
The evening before his untimely death - there was a great dinner - here poor Berthier was complimented by the Russian officers that he stayed loyal to the King and that he did not desert like so many others to Napoléon, this embarassed Berthier a bit.
So the Russian officers regarded Berthier as loyal to the King, why should they even dream an assasination, and contrary to your believe - my opinion about the Russian officers, field army - is that they were not a bunch of criminals - some despised the actions of Figner.
Berthier left Paris with the King
Then he left the King to visit his father in law in Bamberg
From there he was put under "mild" surveillance (he still was able to make strolls etc.)
Then he pledged to return to France to look after his interests
This was denied by the Bavarian King
Then he pledged that his wife could return to France
This was granted but he had to stay in Bamberg
His wife and children were stopped in the area controlled by the Austrians and sent back
Berthier again wrote to the Bavarian king.
The Bavarian kind sent a general to the Austrians to sort out the Berthiers wife will be able to travel to France
Then Russian troops march trough Bamberg
Berthier watches them with his "spyglass" for two days.
Then the dinner happends, Berthier is regarded as loyal subject to the French King
It happened after Berthier sees his children off - who went with the nurse woman for an afternoon walk or coach ride, so he was in the room of his children, alone for a moment.
My motiviation to do research in the archives was to clarify some mysteries of the death of Berthier.
For me - it was an accident, but still the suicide theory stays open, the other theories are fantasy.
To add, in case you are a guest in traditional Bavaria or Franconia in that case, you are as good as sacrosanct. Berthier had close relationship with the Bavarian King, he was even burried in the family grave of his wife and not in Paris, he was regarded to be one of "us".
The Bavarian king would not have tolerated any assasination attempt of Berthier .
yours firm in research