My take on the shepardess thing was that Granny Aching had the same vision of what a shepardess was when she was little, and maybe even thought of herself that way, or least wanted it to be that way. but the reality turned out differently. Basically she was seeing a glimpse of how she used to think, made into a physical thing. which is why she is seen contemplating it on several occasions. And tiffany mistook that as Granny taking it the wrong way. and why Granny appears in that form later.
"The reason an author needs to know the rules of grammar isn't so he or she never breaks them, but so the author knows how to break them."