"The sandwich is not eaten."
Do you need the word 'stato'? Is it okay to just say 'il panino non è mangiato' for 'the sandwich is not eaten'?
Theoretically, you're right.
In practice, you would never say "il panino è mangiato" "il panino non è mangiato" without having written WHO did it.
"Il panino non è mangiato dai bambini" would be ok.
With the passive present we would say "Il panino non viene mangiato". Curious, I know. But passive is a pretty difficult topic, I hope this wasn't a beginner sentence. :/
You can use "venire" to make the passive, or use always the past tense with essere.
Something about the passive here. http://ilgur.com/tag/il-passivo/
I get what you're saying. The English sentence here isn't really one that you would say either. It's not that it's wrong, just weird. You might say 'the sandwich wasn't eaten' or, more likely, simply 'nobody ate the sandwich'. It's the same thing in Italian, or any other language, I suppose. The fact that a sentence is technically correct doesn't necessarily mean that you should use it. :) I will try to remember the 'stato'. Thanks!