Well i think you've thaught about polyjuice more than rowling did. On the whole turning into girls thing, what would happen do oyu think if say someone was concieved while a parent was under the efects of polyjuice potion? Would the rsulting offspring (given a suficient supply of hte potion to allow it to be carried to term in the case of a female) partake of the genetics of the origional parent, or that of the polijuice source.
the more i think about it it the more polyjuice seems like an extra convienent dues ex machina rather than a well thaught out idea.