> The jury can't charge someone with a crime beyond what the prosecution argued.

No, but they can find someone guilty without reference to what the prosecution says.

Neither the jury, nor the prosecution, nor me, nor even (likely) you believes that Manson was convicted solely on the basis of hypothetical speech.

Tex Watson testified that Manson explicitly ordered the Tate killings.

Manson was physically present at the scene of the LaBianca killings, and actually entered the house (again, according to Watson).