First, I find it very strange that you say the comment was "inserted" in Matthew.
Second, all the Gospels differ in that they show the same events from different perspectives - not different facts. This would indicate that Matthew heard the comment whereas the others did not. It does not mean it was not said. Your view implies less information means a more valid point - hardly logical.
Third, you correctly state the Books that address the issue, Matthew, Mark, and Luke. You go on to say the difference is significant because Matthew was written to target a Jewish audience. Hmmm - so was Mark and Luke.
You also state, "What Jesus was teaching the Jews was that they could brake betrothal for fornication." ---- Hmm - needless to say you can not back this up with Scripture.
You continue with, "Thats why it isn't in the Gospels written to the gentiles in my opinion. No such custom." --- Hmm - I think you need to bone up on your history. Many cultures of that region not only had, but still have , that tradition. Additionally, ALL the Gospels are "written" for Jew and Gentile alike.
As to "common schools of thought" there was no such animal. Not only did the Pharisees and Sadducees disagree but at the time of Jesus the Halakah itself was in a state of flux and was not "set in stone". In fact there were many disputes at the time between the Bet Hillel and the Bet Shammai. Jesus often surprised those around Him because He could posit a Pharisees opinion to a Sadducee and they couldn't denounce Him for it. He was that smart! That's one reason He blew them away - He knew their doctrines better than they did!
Lastly, a point which I will grant some may debate. You state, "And finally note that since the OT penalty for adultery was death, not divorce, it would seem inconsistent to recommend divorce as we know it in Matthew considering they were still under OT law." Not true, the OT ended with John the Baptist.
I will respectfully agree to disagree.