Your (the OP's) theory seems a bit random,
but I see what you're getting at. Although, I'm pretty sure that the old woman is truly Link's grandmother. There's nothing in
The Wind Waker to hint otherwise; likewise, there is nothing that hints that she is Impa.
Sure, Nintendo has been known to be incredibly vague and unclear about this sort of thing, but I'm sure this isn't the case with Link's grandmother's identity.
On the other hand, IF it were indeed true, then, why can't she simply be both...as in Link's grandmother and Impa? Impa is known to be reincarnated along with Link and Zelda in most titles. That said, who's to say she can't be reincarnated into Link's grandmother? However, as I said in my previous paragraph, this is more than likely not the case.
If anything, she is probably a reference to the "Old Woman" that appears in some games in the series. Moreover, the Old Woman has been revealed to be none other than Impa in some of the games that she's appeared in. So in a nutshell, I guess we could conclude that Link's grandmother may be a [small] reference to Impa...