Good point. I am amazed at the "she did it" theory still touted.
That is not a really good analysis of the note.
Just as an example, I seem to recall the multiple "she dies" was used in the movie "The Way of the Gun". The movie came out after the murder but I would say it was used then because it is common movie tough guy language since talkies.
The person who wrote the note knowing Jon was already dead does not point the finger at any individual other then the murderer who is persons unknown.
As far as the "and hence" statements made by Patsy Ramsey after the note appeared, see the "she dies" comment. People do that all the time, even screen writers.