In the "argument" of books two through four, Walter and Marten appear to be different people. The argument says something along the lines of Walter being more dangerous than Marten had been.
If it is any help, I have just read the part in DT7 where
So no, I don't think he can be Farson, because he serves him. I can quote from the book I have in my hand. He was also known as Rudin Filaro, when he fought .