At Roadblock 1, should have asserted he is the subject matter expert on React, and the existing team is not. And walked if they did not accept that. You can be

At Roadblock 1, should have asserted he is the subject matter expert on React, and the existing team is not. And walked if they did not accept that. You can be polite and diplomatic when setting hard boundaries like that. Bottom line reality is, if the existing team is so awesome with React, what do they need the author for? He should remove himself on the ethic of Cost Containment. And if the actual reality is true, and the existing team only knows enough to understand React represents a conceptual shift in how to design code and how to complain about it, let the author do his job and the team grows their skills.