Jeppo

Does he also argue for Aramaic primacy?

If by 'Aramaic primacy' you mean the hypothesis that the NT was originally written in Aramaic, then definitely not. My understanding is that Casey attempts to reconstruct some of the possible original Aramaic underlying the Greek translation of (Aramaic speaking) eye-witness reports and the 'sayings' of Jesus.

I've read Ehrman's take on the mythicists, but not Casey's. Having said that I'm currently reading Maurice Casey's Jesus of Nazareth: An independent Historian's Account of his Life and Teaching. It's a very impressive handling of the source material, particularly as Casey knows his Aramaic. You may be interested to note that he posits a much earlier date for Mark (c. 40 CE), and suggests that Mark isn't so much an unfinished gospel as completely unfinished; more of a rough draft, which we know was copied & re-worked by Matthew & Luke. "It contains many features which cry out for revision, including both Aramaisms and other mistakes, such that any decent literate Greek speakers would alter it, as Matthew and Luke did abundantly."

Although Casey's work might not be appealing to those of a conservative bent, by default he makes a very good case for accepting the historicity of the gospels (with the exception of John's gospel). Casey really knows his stuff - whether or not you agree with his conclusions - and as an agnostic scholar he ought to be required reading for any atheist who might be put off by 'christian' scholars.