I also wouldn't worry about too much if your stats score is B or B+. I believe many grad schools would expect the applicants to have some exposure to quant stuffs but not necessarily to excel in advanced statistics, unless your subfield of interest is political methodology. I would suggest to make sure about the mixed-method paradigm is highlighted.
In terms of writing sample, I am not sure whether it is smart to show basic stats if that is not your strong suit (Unless you have enough room to show your qual strength, or mixed-method way of thinking). As far as I know, in terms of empirical works, faculty tends to figure out how you arrive at the methods for your research question/hypothesis, instead of what method is being applied - what is your "data" (in a broad sense), how you justify the method selection, how you interpret the results etc.