Q: "Why does Paul mention James before Peter (Galatians 2:7-9) and why is Peters' name used as "Cephas" instead sometimes?"
Firstly, if the hidden assumption behind the question here is, “Any name mentioned first in a list of names necessarily means that the person holding that name mentioned first is superior to all others in that list” then we should recognize that the assumption is not always true. Now, of course, Peter’s name is mentioned first when it comes to the listing of apostles, and that’s a case to be made for his superiority among the apostles. But, this type of conclusion is made with (1) the additional biblical and historical data that shows Peter was in fact the leader of the apostles, and (2) the fact that Peter is consistently mentioned first not only one time, but many times over and over, making it more likely. Second, I want to recognize that this is a good question, and we can actually see the logical pattern as to how Paul lists the four people (me [Paul], James and Cephas and John), as opposed to being merely random. But before that, it is important for us readers to recognize ...