In the book it is also stted that some historians see Zita's version as a way to clear the Habsburg family from any flaw. I assume that the Clemenceau error was prety obvious, to obvious for Zita to make it, so maybe she meant the later PM Clemenceau?
I think that 'person unknown´ could have lead to speculations etc and an official investigation while this way everything could be silenced. If Zita's version is true (not likely), and the emperor wanted to evade an internationl incident wih France, this would have been the most logical thig to do.
The other points which you state are indeed not in favour of Zita's version, but I do not think Zita was a person who would make things like these up.
With a bit of imagination the other points you state could have been set up as well, though a bit far-fetched. So a suicide is the most logical explanation (thnkig about the other posibilities is much more interesting though
)