The context isn't about the sabbath, but about the time that Christ spent in the grave. The reason for quoting the verse was to show that there is a scriptural precedent, from the mouth of Christ, to show that he can and did consider a day and a night more than just "part" of day and night.
God didn't command Sabbath keeping for the whole world, just the Jews in Israel.
You're right in that he didn't give the sabbath commandment for the whole world. He only gave it to all of those who wish to worship the one, true God. This included Israel AND gentiles:
Exo 20:10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
That sabbath commandment is the only one of the ten commandments that God said specifically that non-Israelites, strangers, should observe.
There was no time conflicts for the Jews of Israel when this was in effect.
The sabbath varies, as you pointed out, from place to place. Wherever you are, at sunset, on the 7th day, begins the sabbath. There is scriptural precedent for this in the book of Daniel where Daniel was in captivity in Babylon and kept the sabbath according to the local sunset on the 7th day.
You don't run into these conflicts with moral laws, just the ceremonial law of the Sabbath.
Sure you do. It's called "moral relativism" and it's often the justfication for breaking all of God's commandments. And the sabbath is a moral law.