Result! I’ve now got an idea of where to look for the lost full text of Eusebius’ Quaestiones ad Stephanum/Marinum!
When Angelo Mai published the sad remains of this work in 1823, he added a note that Latino Latini (in the 16th century) said that Cardinal Sirleto had told him that he had seen a manuscript of this work, in three books, in Sicily. Migne reprinted this.
But I suffer from acute reluctance to repeat stuff unchecked. For a year now I have been trying to locate a copy of the letter in which Latini said this. Today I succeeded. And … it turns out that Mai has misled us all. He did not mark insertions and omissions into what Latini wrote. Here is an excerpt:
Scire etiam te vult in Sicilia inventum esse Eustathii Antiocheni Episcopi librum de mundi creatione, idest de sex dierum operibus, unde Basilii plurima videantur sumpta esse; praeterea libros tres Eusebii Caesariensis de Evangeliorum diaphonea, qui omnes, ut ipse sperat, brevi in lucem prodibunt.
He also wants you to know that in Sicily there have been found the book of Eustathius bishop of Antioch on the creation of the world, that is of the works of the six days, from which many things seem to have been taken by Basil; in addition three books of Eusebius of Caesarea on the divergences in the Gospels, all of which, so he hopes, will be brought into the light shortly.
Now this is an important difference. For the first time we learn that the main find is a volume of Eustathius; and, if we translate praeterea as “following it”, it suggests to me that the two ‘finds’ are in a single physical volume. If so, it becomes no mystery that the Eusebius might “disappear.”
When cataloguing a pile of manuscripts, the lazy librarian flips open the cover, scribbles down the title of the first work in it, and then closes the book and moves on to the next one. And if there is one characteristic endemic to the Southern Italian librarian, it is laziness. And who in the world would trouble themselves over a volume of Eustathius? My eyes close, almost at the name. How likely is this to be interesting? Not very. How likely is it that anyone has examined such a volume in centuries? Not very.
What this means is that we ought to be looking in the manuscript catalogues for a volume of Eustathius. If there is one, say in the old Royal Library in Naples, the capital of the Kingdom of the Two Sicilies, it may well repay investigation!
“You will find,” said Martin Routh, “it is a very good practice to always verify your references.” Wouldn’t it be something to rediscover that manuscript!!!
UPDATE: I have tossed an email of enquiry to the Biblioteca Nazionale in Naples, and also to the Biblioteca centrale della Regione siciliana. They ought to know what they have. If they reply.