James is not making up some scripture. He is quoting Amos 9:11,12. Look at your references. He is interpreting the Old Testament promise in the light of the New Testament reality of what God is doing. "After this" is part of the quote not a reference to "after" the Gentiles are made a people. Study your rules of hermeneutics. (that's 'biblical interpretaion' for the intelligent)
Where are the words "after this" in Amos 9:11-12? Oh I get it. When Amos says "In that day", it means "after this". Is that what you are saying.?
I hate to burst your hermeunitical bubble -- but those words of James -- they are Scripture. They are not just an interpretation of Scripture -- they are Scripture. And James is telling them that what Amos and the others prophesied regarding the restoration of the tabernacle of David could not take place until after God had first taken out a people for his name.
Any sixth grader with no preconceived theological notions, reading the words on the page can tell you exactly what they say and what they mean.