If we want to say "In Spain, paella is eaten" , then "se come" would be the obvious answer. When the sentence is "In Spain, people eat paella" there is a nominative word here. People. There is another translation for the third person plural in this case, no? Or would you have to explicitly say "En España las personas comen paella?" I'm sure I have seen uses of "se comen" without an explicitly named plural subject...

With comer, there is also the fact that both comer and comerse are possible. So it might be:

En España se come paella.

En México se comen frijoles.

(both "passive se" of comer)

But in theory, would it also be possible to use comerse with impersonal plural:

En mi aldea se comen una manzana cada mañana = En mi aldea la gente se come una manzana cada mañana?