"After this [after visiting the Gentiles to take out a people for his own] I will return again and build the tabernacle of David which is fallen down and will build again its ruins and will set it up." [Acts 15:16]
After this -- AFTER THIS -- AFTER THIS
When James spoke, the tabernacle of David was not in ruins yet. It was yet future.
Regarding Amos 9:11-12 says nothing of the sort. As a matter of fact Amos says that the restoration of israel will take place so that "they may possess the remnant of Edom, and of all the nations that are called by my name"."
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)