As a minor note, I think you mean "Hellenic", not "Hellenistic" (the latter is used to denote the period after Alexander and before the Roman conquest, ca. 323-31 BC, if we take the Battle of Actium as the end date for this period).

But regarding your question, the single best resource currently available on this goddess is Susan Deacy's book, Athena, published in 2008 by Routledge as part of their series, Gods and Heroes of the Ancient World. I think the paper version is currently out of print, but Kindle/e-book versions should still be around. If you're interested in Athena, it's worth trying to get your hands on a copy.

Deacy devotes an entire chapter to "Tracing Athena's origins" (pp. 33-44). Tellingly, she starts this chapter by citing Jeffrey Hurwit, who writes in his book The Athenian Acropolis (1999), p. 12:

We do not know where the Greek gods came from, but the conventional view is that most of them came from somewhere else.

Deacy starts her chapter by stating that question of origins isn't something that academics focus on, since hunts for origins -- real or perceived -- are often dead ends. Instead, the focus is usually on how something functions in a context for which we do have sufficient information (which is why the next chapter deals with Athena's place in the Greek pantheon). Still, she tackles the issue by discussing the links between Athena and the matriarchy movement that was common once (and in certain circles still is!), linked to notions of a prehistoric Great Goddess, and so on, before showing that there is reason to doubt the existence of a primordial matriarchy.

As Deacy notes, p. 40:

We need only look at historical societies to realise that images of powerful females hardly provide evidence for matriarchy, the proliferation of female imagery from fifth-century Athens being a case in point. Women were notoriously marginalised in that society, but females are prominent in its art and literature, including the transgressive Amazons and Klytaimnestra and above all Athena. We might also adduce the adoration of the Virgin in Roman Catholic countries as further evidence that venerating female figures need not denote matriarchy.

Your question suggests that Classical Greece being patriarchal is at odds with them worshipping a female warrior deity, but one clearly does not exclude the other. As far as Athena is concerned, it seems likely that the Greeks -- or whoever first came up with her, if she has Minoan roots, as some have argued -- took inspiration from the cultures of the ancient Near East, where female warrior deities were common among e.g. the Hittites (Arinna) and the Egyptians (Neith), as Deacy explains on pp. 41-43.

Regarding Athena's origins, Deacy concludes, pp. 43-44:

The desire to identify Athena’s origins continues to excite interest, but there has been no wholly satisfactory solution. [...] The theory that her origins can be traced to a hypothetical matriarchal prehistory looks outmoded from a scholarly perspective, as does the notion that she was an amalgam of a peaceful prehellenic deity and the war goddess of the invading Greeks. Athena shares characteristics of numerous older warrior goddesses, including the Hittite Sungoddess, but there is insufficient evidence to enable any conclusive conclusions.

As regards your statement that the ancient Greeks had little evidence for "shield maidens", there is, of course, some: Telesilla of Argos is one (historical) example (Pausanius 2.20.8-10), the heroine Atalanta is a mythological one. The Greeks also told stories about the Amazons, who were warrior-women, so they were certainly not unfamiliar with the concept of women fighting. And Athena, in many ways, is the quintessential "shield maiden".

I hope that answers your question. Again, if you're interested in the topic, you should try to get Deacy's book.