Is 56:6-8 gentiles (not Jews) are specifically singled out for Sabbath keeping
Not true. They are gentiles. "Foreigners". And it does not matter where they live according to Isaiah 56.
“Also the foreigners who join themselves to the Lord,
To minister to Him, and to love the name of the Lord,
To be His servants, every one who keeps from profaning the Sabbath
And holds fast My covenant;
7 Even those I will bring to My holy mountain
And make them joyful in My house of prayer.
Their burnt offerings and their sacrifices will be acceptable on My altar;
For My house will be called a house of prayer for all the nations.”
Ex 12:48 does NOT say "anyone who keeps from profaning the Sabbath is a Jew". And we all know it.
Ex 12
48 But if a stranger sojourns with you, and celebrates the Passover to the Lord, let all his males be circumcised, and then let him come near to celebrate it; and he shall be like a native of the land. But no uncircumcised person may eat of it.
Nothing at all there about the weekly Sabbath - the Sabbath commandment. As we all can see clearly.
So it is gentiles .. foreigners "all nations" in Isaiah 56 -- not circumcised passover-keeping Jews.
And it is foreigners - "god fearers" in Acts 14 - gentiles that are worshiping in the Synagogue "Sabbath after Sabbath" and asking for MORE Gospel preaching "NEXT Sabbath" when the "entire town" shows up... still gentiles.
Thus it is that gentiles show up in Acts 17:1-4 and in Acts 18:4-6 Sabbath after Sabbath in the synagogue for Gospel preaching.
The text you reference only speaks about Passover and circumcision ... nothing at all in it about gentiles in all nations and the weekly Sabbath.
By contrast - the texts I point to - do reference gentiles in all nations and the weekly Sabbath. and we see them not only in Isaiah 56 but also in the NT - in Acts 13, Acts 17, Acts 18 -- Sabbath after Sabbath... gentiles, not Jews
Isaiah 56 does not say the foreigner lives in Israel. Those gentiles keeping Sabbath in Acts 13, Acts 17, Acts 18 were not in Israel, were not circumcised, were not Jews.