Interesting article! Here's what I think:
-To me, Willow's outfit in WTTHM didn't seem particularly frum to me, at least not in the uniquely Orthodox sense. If anything, I saw it as sort of a nod to the traditional Catholic schoolgirl outfit. Not that that's what Sheila Rosenberg was going for, but I guess what I mean is that it was just conservative, in general. Fitting choice for a mother who wanted her daughter to be covered up and traditionally dressed. But of course if you pair that outfit with the hints Willow has dropped about Ira Rosenberg I understand how it seems like she came from at least a conservative household.
-I think that though her parents might have been conservative, Willow herself isn't by any means a practicing Jew. I could be wrong of course, but since we never see any religious practices alluded to on the show after she leaves her home, that seems to be the case to me. Of course in season 7 however, she puts a stone on Tara's gravesite which is of course a Jewish tradition but doesn't really bear an indication of religious observation as much as it is perhaps merely a nod to her culture.
-I would actually argue that once Willow refers to herself as a "wicca," (which I think she does, right? Not just a witch?) she is embracing the practice and any theology that might come along with it. I would think that a lot of observant Jews would consider her to have violated a lot of the basic laws of Judaism simply by casting spells which call upon "gods" or "goddesses." Spells to Osiris, Hectate, Diana, etc, are all essentially examples of worship or at least acknowledgement of other deities and an expectation of services at their hands. I know you acknowledge in your article that magic is of course inherently in opposition to the ideologies of Judaism but I think because of this many people would say that she can't be Jewish; at least not halachically speaking.
The bottom line for me is this: the show itself forces its characters to acknowledge the existence of other deities, of demons, and of magic. Basically, there is a gigantic, inherent conflict between embracing any of these hellmouthian truths and subscribing to Judeo-Christian beliefs by practice and observation. I think it would be really interesting to consider how someone (maybe like Willow) who comes from a religious background comes to terms with a world like Sunnydale, and whether they are able to maintain their previous religious practices. And if so, how? In effect if they continued to practice, they'd be coming up with sort of a different system altogether...