OK...honestly trying to understand here...
You can't really mean that at least one item of EVIDENCEhas to be proved beyond a reasonable doubt, because evidence doesn't get proved...evidence is what you USE to prove FACTS. So do you mean that the jury must find, from the totality of whatever evidence is presented, at least one FACT proved beyond a reasonable doubt that supports each ELEMENT of the crime?
In other words, are you saying, for example, that if the jury has to find premeditation as one element of one count, they have to find beyond a reasonable doubt that at least one fact exists in support of that element? If so, that makes a lot more sense than what we've been talking about up until now.
The jury will decide what evidence, if any, presented at trial represents a fact proved beyond a reasonal doubt. The trial judge will instruct the jury that they are the sole finders-of-fact. Certainly, both sides can agree to stipulate something as fact during the course of a trial, however, the jury usually determines the large measure of what evidence is fact for the purpose of deciding "guilty" or "not guilty".
As regards the first-degree murder charge against Casey, the judge will instruct the jury that, for any of the charges, the People must prove beyond a reasonable doubt every essential element of that charge. There is no direct evidence that we know of in this case, which means that the proof for each essential element of murder one must necessarily come via an inferred conclusion, and -- as we just went through -- any facts the jury relies on to support an inferred conclusion of "guilty" must be proved beyond a reasonable doubt.
For example, as regards the murder one charge, I expect the judge to instuct the jury that intent (willful), premeditation (planning, perhaps reflection), deliberation (consideration or perhaps reflection) and malice (malice aforethought or implied malice) must be proved beyond a reasonable doubt.
Please note that I do not believe Florida's model jury instruction for murder one will be copied and pasted into the final jury instructions. Florida's standard jury instruction was discussed on another thread, and my rememberance is that Florida's standard instruction did not specifically state intent (willful) or malice (malice aforethought or implied malice) as a requirement (essential element). Florida might sweep intent and malice under premeditation, -- diiferent states use different terms for these four basic elements. I expect the trial judge will greatly clarify each term that Florida currently says are essential elements of murder one.
Be that as it may, whatever they might eventually instruct the jury to be an essential element of murder one must be proved beyond a reasonable doubt.