What does this mean: 更不是被告? Does it mean "not the defendant"? The translation I have available (which isn't mine) is the following: "let alone the defendant", so which of us, if any, is correct?
"not the defendant" is just "不是被告"

"let alone the defendant" in Chinese is "更不必提被告"  or "更不用說被告"

"更不是被告" means "specifically not the defendant"

Example:

他不是被告 = he is not the defendant

次被告也判死刑, 更不必提被告 = even the second defendant is sentenced to death, not to mention (let alone) the defendant. -- of course he has to be sentenced to death

犯案的不是陳某或張某, 更不是被告 = The one who committed the crime is not Chen or Zhang, certainly (specifically) not the defendant.