I know the Jesus Seminar held that Q and the Gospel of Thomas were prior in composition to the writing of the Synoptics. Is this still a dominant view among scholars (as far as Thomas)? What can be said for or against this view? And under either scheme, do any of the writings have a literary dependence upon one another? How do they relate?
This question is hotly debated, and there's no real consensus. There's a very recent monograph on exactly this topic, Goodacre's "Thomas and the gospels". He argues strongly that Thomas is dependent on Matthew and Luke. According to Goodacre in an interview with his publisher, scholars are divided roughly 50/50 on whether Thomas is early and independent or late and dependent. Goodacre should be well situated to give such an estimate, since writing such a book requires reading the relevant literature, and furthermore his claim is consistent with the wide range of opinions cited at earlychristianwritings. So there certainly isn't a dominant view among scholars, and there may not even be a clear majority view.
The conservative view is that the Gospel of Thomas dates from the second century. This is largely because it demonstrates the existence of Gnostic Christian beliefs and, to some, it is inconceivable that such variant beliefs could have existed so soon after the crucifixion of Jesus. On the other hand, the dominant view among critical scholars is that it is concurrent with Q, and possibly somewhat prior to Mark.
When GThomas was first studied, it was suggested that this was, at last, the hypothetical 'Q' gospel, used as a source by the authors of Matthew and Luke. It was soon realised that it contained passages quite different from what was expected by a reading of the synoptic gospels. That meant that they were two different documents in spite of their similarities.
In both documents, the focus is on the sayings of Jesus, and neither Gthomas nor Q mentions his death or resurrection. Obviously, the communities that used either of these very early gospels for purposes of worship were content that the significance of Jesus lay in his words alone. This challenges the traditional assumption that the early church made Jesus' death and resurrection the essential feature of Christian faith.
Euan Cameron in The Secret Gospels of Jesus, page 5, points to the primitive Christology in Gthomas:
John Dominic Crossan says, in The Birth of Christianity, page 244, that the Q Gospel is much more organised than the Gospel of Thomas in its general structure, and it is not so much verbal or formal associations as topical connections that dominate the composition.
Crossan proposes a 'Common Sayings Tradition' document, written in the Greek language, that was the predessor of both Gthomas and Q, as well as of some sayings found in Mark's Gospel. He says (ibid, page 255) that the original Common Sayings Tradition contained neither Gnosticism nor the apocalypticism of Q, but required redactional adaptation toward either or both of those eschatologies.