In reply to your other point about Onesimus possibly not being a Jew, I would reply that Christ said all believers would be grafted into the true vine which is Israel, so in effect, Onesimus became a Jew when he accepted Christ was the Jewish Messiah.
I agree that Onesimus was a spiritual Jew, as indeed are all Christians.
However, the author of Philemon himself made it very clear that Christians were not bound by the Jewish Law.