You wrote, "Duke Wilhelm was Berthier's father-in-law, not his father. " Thanks, misread the source.
"So, what precisely is this information/source that tells you all these fascinating details about the Russians assassinating Berthier because they had met with the duke and knew the layout of the palace beforehand and were able to waltz back in without a challenge? "
I make -nor have I made- any claims about any special details concerning this assassination. What I do know -with some background on the subject- is the nature of the Russians. One distinctive characteristic of the Russians is that there have been times when elements within have embraced assassination as a political tool. One of those times was 1812-1815 and the target of these assassins was any French soldier within reach. To say that the Russian Army was bunch of assassins would be going too far but there were certainly a good percentage dedicated to killing French soldiers -any French soldier. There would have even been some to whom the thought of letting N.'s number 2 man -who was in their reach- live even another day would have been a humiliation. To many, letting Berthier escape their reach and flee back into the arms of N., a la Ney, could never be allowed. Most likely, in the eyes of the Russians, Berthier signed his own death warrant when he requested a passport back to France.
Berthier's Fable: If a Bear and a chicken are put together inside a large barn and the next morning the chicken is torn to pieces, there's good reason to suspect that the Bear might have had something to so with it.
"What source tells you Berthier was not in his own house at the time? "
Outside the wall of the Bamberg Royal Palace (the New Residence) there is a sign denoting the spot where he hit the ground. The New Residence was his father-in-laws (thanks, again.)