On "Aion" vs. "Aeon", the first sentence of your quote from Luedke reads:I didn't search for "Aion", the author Luedke did. Naturally he used the Greek "Aion".
So I naturally thought he searched for occurrences of "aeon". Also, there is a problem: Gnostic sources use "aeon" (or "aion" or its Latin or Coptic equivalent) all the time, at least in English translation. Your quote from Luedke didn't mention them at all, one reason why I brought up the issue of the spelling. Finally, the main source for Gnostic terminology in the West, until I don't know when--late 19th century at least--was Irenaeus in a Latin translation; the Latin term corresponding to "aion" is "aeuum" (http://en.wikipedia.org/wiki/Aeon), at least in the meaning of "age". I that might have been the spelling used by Irenaeus (but I don't know; it might have been "Aeon"). If so, that is another term that would need to be searched."Aeon" kommt in griechischer Literatur vor: 8.Jh.v.Chr. 22mal, 6.Jh. 60mal, 5.Jh. 131mal, 4. Jh.
68mal, 3.Jh. 26mal, 2.Jh. 65mal, 1.Jh.v.Chr. 197mal, 1.Jh.n.Chr. 588mal, 2.Jh. 1855mal und
3.Jh. 338mal. Damit ist das 1. und 2.Jh.n.Chr. die Blütezeit dieses Begriffs.
On "Seth", is there any other reason for thinking there is a relation to the Hebrew "Seth", beside the similarity of the names and the fact that the Jews spent some time in Egypt? The Egyptian god Seth, of upper Egypt (desert) was considered evil by the followers of Osiris in lower Egypt (or maybe just their Hittite conquerers). So it is possible that Jews were considered worshipers of Seth by those who wanted to demonize Jews. But did the Jews themselves consider themselves worshipers of Seth? If not, it doesn't matter what others thought. I seem to remember a calf, which might be related to Osiris and his sacred bulls.
What might be interesting would be if you could find parallels between the Gnostic Sethians and the myths and rites of the Egyptian worshipers of Seth. Otherwise, it won't do to use the Egyptian worshipers of Seth as evidence for the roots of the Gnostic Sethians. I would think that there is enough material in the Gnostic Sethians by themselves to occupy you, at least to start with. For example, I think the Pistis Sophia is probably more Sethian than Valentinian, since it features "Barbelo", a term the Sethians used but not the Valentinians.
You also need to consider our knowledge, or lack of it, of where the Gnostic Sethians originated. It's not enough to say that some 4th century Gnostic monks in Upper (i.e. southern) Egypt had a library with Sethian books in it. It may have originated in Palastine, Syria, or points north or east. If the Valentinians originated in Egypt, that does not mean the Sethians did. Both movmenets have much in common with movements outside of Egypt, e.g. Simon Magus. The Wikipedia article says that the Sethians existed as far away as Greater Armenia, even if by 375 c.e. they only existed in Egypt (http://en.wikipedia.org/wiki/Sethianism).
The Wikipedia article also lists 9 Sethian documents in the Nag Hammadi collection. That is out of 45 documents in all, counting a bad translation of a selection from the Republic and the Middle Platonic Aesclepius (a Coptic translation). We have to remember, too, that the Nag Hammadi documents are not actually "original"; they are translations into Coptic, of better or worse quality, from the Greek. ("Coptic" means: Egyptian spoken language written with Greek characters). The main overlap between Irenaeus and Nag Hammadi--the one in Latin, the other in Coptic--that I know of is the Apocryphon of John, a Sethian document originally in Greek.