To be honest it sounds like you are in a really strong position because the external doesn't know much about your subject area but has a personal passion for innovative research methods. One of the joys of qualitative research is that it's all about the interpretation - YOUR interpretation - and as such you are the expert on what the data has to say. If the research findings justify the methods then that's a good enough reason for choosing a particular approach over others.

It might be worth spending some time before the viva reflecting on what your research would have looked like if you had used alternative methods, if you haven't included that sort of stuff in your conclusion. I read a book recently which I thought was really imaginative in justifying their choice of methods because it explained what would have been missed if the conventional approach had been used.