The Earliest Witnesses to De Recta in Deum Fide

The strongest argument for thinking that the original text of De Recta in Deum Fide only involved a single heretic - Megethius the Marcionite - is because this is what our earlier sources tells us.  From Pretty in the introduction to his translation:

Practically all writers of ancient and mediaeval times refer to the author of the Dialogue under the name of Origen. The earliest mention of the work is in the Philocalia of Origen, which is a selection of passages from Origen's writings made by St. Basil of Caesarea (ca. 330-379) and St. Gregory of Nazianzus (ca. 329-389). At the end of chapter 24 stands the following:

The preceding has been drawn from the Preparation for the Gospel of Eusebius of Palestine, Book 7. He says it is from Maximus, a writer of some importance among Christians. But exactly the same words have been found in the Dialogue of Origen against Marcionites and other heretics in which Eutropius is adjudicator, and Megethius the opposing speaker.

Assuming that Basil and Gregory are responsible for this statement, it can hardly have been written later than 379 (the death of Basil) and therefore our Dialogue must have been composed earlier than this. The Philocalia declaration also makes it clear that towards the latter part of the fourth century, the work was attributed by some to Origen.

The next witness is Rufinus (ca. 345-410), who probably made his translation of the Dialogue soon after 387. If one accepts his was the initial inscription (see supra), he definitely claims Origen as the author. However, the next writer to refer to the book, Theodoret, Bishop of Cyrrhus (ca. 393-458), makes an important distinction. In the preface to his Compendium of Heretical Fables he says, that he had "collected these fables of the ancient heresies from the ancient teachers of the Church — Justin, Origen, and Adamantius. Here Adamantius is given a place separate from Origen. Another certain reference is found in the Guide of Anastasius Sinaita (d. ca. 700). Among other material, this book contains 154 Questions and Answers. Although Anastasius is thought not to be responsible for their present form, their main substance no doubt goes back to him. The answer to Question 48 is "Of Origen, from the Dialogue against Megethius the Marcionite". There follows a somewhat free translation of a passage from the First Part of the Dialogue (from sect. 818c). [p. 9 - 10]
 

I think this is enough for us to begin our investigation.  Whenever I hear that an ancient author has a 'loose citation' of an existing text, my ears perk up because I begin to suspect this is a more original version of the material.  To this end, I find it at least possible that Anastasius is citing from the original text of the Dialogue - one directed only against Megethius the Marcionite - and that the circle of Origenists 'corrected' the original text in order to make their spiritual master Origen seem less heretical.