Is there a difference between "Who necessarily do not exist" or "who do not exist necessarily"?

Yes, there is a difference.

"Who necessarily do not exist" means that those people do not exist due to some need or requirement. This is saying the "who" do not exist.

"Who do not exist necessarily" means that those people may not exist, that we can't logically conclude that they do exist. This is NOT saying that the "who" do not exist. Instead, it is saying that the "who" may or may not exist.

The modality referred to is occasioned by the adverb necessarily. Apparently, Eco is considering two different statements, where the negation either does not, or does, span the modality, respectively:

• necessarily (not exist)

• not (necessarily exist)

These would indeed be understood differently (provided our understanding of "necessarily X" is different to "X" alone, of course. This may take some mind bending :))

The intuitive understanding (of some eminent philosophers) of the modality of necessity led to semantic models for modal statments, such as possible world semantics. Under this semantics, necessarily X is explained as "X holds in all possible worlds"; this explanation makes apparent the distinction of the two statements.

Philosophers' preoccupation with necessity (and the consequent need to develop a semantics for it) traditionally stemmed from considerations of a priori truths or ontological arguments, etc.