Yes, "death by defenestration has a venerable history"
I knew it was popular back in the good old days; however, defenestration also has a more recent history of being used as a means of covering up what was a mob hit. Just as in 1815, it can be difficult to determine if it was self inflicted or not.
It's that history that makes me think first about assassination rather than accident or suicide. Defenestration has the advantage in a situation like Berthier's of creating a question of was he assassinated or something else. Because of the sensitive situation of being in an allied country and in the Royal residence, an out and out assassination would be a problem, so, like any good politically-aware hit, it would have had to be handled with care.
To me, it's baffling that the possibility of assassination is so lightly dismissed by some, especially considering who were some of the last people to have been in contact with Berthier before his death. It wasn't just a bunch of nondescript Russian officers that he met with, instead it was Russian General Sacken. Sacken has an interesting history that includes a childhood lived in poverty, he rose from corporal to General and was made a prince for his aggression in battle. He was part of the Battle of Katzbach where thousands of French soldiers were forced into the river and drowned. So, from appearances, Sacken was a very tough character.
Berthier, who according to the German paper, was, especially when not in direct contact with N., prone to get nervous and confused. Berthier's last dinner must have been a very nervous occasion for him. Sitting down with a group of tough Russians who were wholly dedicated to defeating N. could not have been an easy thing. The only report we have about what was said is that the Sacken complemented Berthier on his loyalty to the King when so many others went back to N. and Berthier is reported to have been made nervous. Some seem to think that Sacken was making a statement here. More likely, this was a question or test. To a smart interrogator, Berthier's reaction would have told the truth about where his loyalties really were. A nervous reaction can not be seen as a positive result.
What may have happened can only be guessed at, unless perhaps somebody can dig up a Sacken memoir, but, for a tough character like Sacken it might be a simple thing. In even modest tones it could be made clear to Berthier that he was a dead man. The only issue being the means. A hint that if he was still alive in 24 hours, that, unfortunately, the rest of his family would have to be involved could easily produce the desired results. With Berthier apparently being a nervous person when not in the company of N., just the right threatening look from General Sacken could have delivered the message.
With Berthier indeed dead within 24 hours of meeting Sacken, the Russians of course would have been present to honor the brave and noble French Marshall at his funeral.
Of course, this is nothing more than speculation but it does fit the known facts and is much more reasonable than the strained assertions that the Russians couldn't possibly have any interest in seeing Berthier dead, or the even more bizarre sounding "too much honor" to harm a Marshall of France.
From the beginning, my interest in the defenestration of Berthier had little to do with "solving" the case but instead it was an interest to understand the bias prompting the dismissal of the Russians as a possible suspect. And it can only be called bias to summarily dismiss the last man known to have seen Berthier alive: Sacken.
Bottom line: An investigation that dismisses any possible role of the Russians is -at the very least- incomplete.