One more point. Let us assume Philemon was a Jewish Christian who would have followed the letter of Jewish Law on the subject carefully.
There is no reason to believe Onesimus was a Jew, and the OT allows lifetime and hereditary slavery for non-Jews.
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.