Wow. I think you misconstrue the Strong's entry, as linked to.
a race (as of the same habit), i.e. a tribe; specially, a foreign (non-Jewish) one (usually, by implication, pagan):--Gentile, heathen, nation, people.
(The "as of the same habit" is because the writer thinks the word is related to Gk word for "custom" or "habit".) I am greatly concerned that you present this as saying that tribe is the preferred meaning, to lay a foundation for saying that the commission is to Israel, when at least bu MY reading the article is as much as saying the word is used "specially" of gentiles, and this reading agrees with my general impression and with other sources.
I commend to you the article on ethnos in Gerhard Kittel's magisterial Theological Dictionary of the New Testament Volume II (which does words from Delta to Eta). Here the sense is that the use of ethnoi/ethnoi in the LXX and the NT is far and away predominantly in reference to Gentiles (while laos seems usually to be used for the Jews). The more than 7 page discussion is, as this Dictionary almost always is, really, really interesting.
One of the hinges of your argument is that you suggest Jesus and the twelve confined their ministry almost exclusively to the nation of Israel. When it is suggested that the pre-Resurrection ministry of Jesus and the disciples differs in important ways from the post-Resurrection ministry, the meaning of the word "ethnos" becomes important because either a commission is to go out to all the tribes i.e. : of Israel or to all the nations i.e.: of the earth. Strong doesn't make ethnos as "tribe" case. "Kittel" (or the contributors to the article in question) argues against it.
I think you have a distinctly uphill task to establish a Saturday worshipping largely Jewish in background church as "authentic".
What then, would be your response to Paul? [Galatians 2:7] But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter.