Proto-Granny is so very strange! I don't really have anything to say other than what you've already said here so well, but proto-Granny always throws me for a loop. The Witches books are my favorites, but I am not quite sure I will ever reread this one; I don't feel like he's quite hit his stride with them yet.

Also I think you may be missing a 'not' here?

It's that we don't need books about adolescent girls figuring out if they can be powerful, but it's so much rarer to find books about grown-up women who are powerful.