Not sure what your point is, Greektim. My choice of translation, seated himself, is neither required nor precluded by koine Greek grammar. The NASB translators chose to translate our word in Matthew 23:2, just as I did in John 19:13.
Reflexive pronouns like himself, themselves, reflect the action back on the subject. However, they are sometimes used with active voice verbs, in scripture, i.e. 1 John 5:21. So my offering is both consistent with a translation choice of the NASB (at Matthew 23:2), and of some Koine Greek writers, i.e. John (at 1 John 5:21.)