Sorry, but you don’t seem to understand the timeline. Go back and reread Philemon with an eye towards the fact that Paul was counciling one Christian to accept another Christian as a brother, even though the first was a slave to the second.
First is the fact that slavery at this time was different than American slavery, also known as Chattel slavery. These slaves, as outlined in the Old Testament, could only be held for six years and then had their freedom. Those that wished to, could remain beyond this time but were considered servants for life after that point. Onesimus was one of these “permanent” slaves and was not being held against his will. His crime, aluded too in verse 18, was that he made a vow to remain beyond the six years and then broke that vow. There were penalties for this, which Paul was stating that he would pay so that Christian unity could be restored between these two.
Second, I agree that some of the slaves were fellow Christians to the slave owners in 19th Century America, but I also point out that when this happened, it often resulted in the owner releasing his slaves because they were his fellow Christians.
Highly unlikely. I understand this is what the OT required, but it is generally agreed the Jews had abandoned the practice long before this, just as they had abandoned similar rules for land ownership.
In fact, quite a few scholars contend these rules were never implemented, but rather were held up as ideals.
Both Philemon and Onesimus had Greek names. While this is not dispositive, it at least makes it more likely they were gentile converts rather than Jews. And Paul himself had made it very clear Jewish Law was not binding on gentile Christians.
Philemon lived in Colossae, a Roman/Greek city in Asia Minor. Jewish traditions with regard to slavery would have had no force of law there, assuming they had such force anywhere.
The relevant law code was either Roman or Greek, in which slaves were quite literally consider "animals that speak," and had almost exactly the same status as livestock. Under this law code owners could legally have a slave crucified on a whim. If an owner died mysteriously or was killed by one of his slaves, all his slaves, in some cases many hundreds, were executed.
BTW, if southern slavery was based on biblical precedent, what happened to the whole freedom after seven years bit?
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.