If they're not guilty enough to serve the sentence, they're not guilty. I don't think anyone should be convicted of a crime that the jury doesn't think they committed. Period.
I already stated that one can be convicted on circumstantial evidence.
We are simply talking about sentencing guidelines and not about guilt or innocense. The legislature can determine any sentencing guidelines it wishes. It requires 2, 5, 7, 15, life, death, etc. based on the details of the crime.
They can pass a law that limits death sentencing to those guilty parties that have either 2 eyewitnesses and/or physical evidence.
Or they can have a new trial called 1st degree murder capital that requires 2 eyewitnesses and/or physical evidence. It can be the only trial eligible to impose the death sentence. If the prosecution doesn't have the requisite evidence, then they will prosecute under 1st degree murder life.
These are simply my opinions. They are designed to provide something near absolute certainty that innocent parties are not executed.