Colossians 2:16
Notice something? 'Sabbath days', not Sabbath day. That already, logically, by the way, notes why there was a sunday Shabbat, in the first place. Because they already had it. The details of observance are moot to this argument, and we know that they had a sunday Shabbat, and it is called the Lords Day. So, where are you even getting all this speculation from?
That is pretty weird that they would arbitrarily change it to a different day, don't you think? Your argument seems too speculative.
Where is the logic? The resurrection occured before your theoretical 'change', of days, it's random speculation.
'Gentiles', aren't Christians. Gentiles is used as a sole descriptor for non Jesus followers, in the New Testament. There are basically three groups, which by words are used,
Gentiles
Jews
Jesus followers.
The Church in the New Testament, is called the Church of God, once "gentiles", become Jesus followers, or members of the Church of God, they are not called Gentiles, anymore. Jews also are not called Jews anymore, when they join, thusly why those descriptors are used.
In other words, they used those descriptions because it's practical, and denotes three separate groups.