As far as I've discovered, though Isis was the goddess with "a thousand names", no epithet with a "virginity" aspect was ever noted before the similarities were noted between her and the Virgin Mary. Some have suggested that Isis was a virgin because of the myth describing when she discovered and magically assembled Osiris' body parts, scattered by Seth. Isis failed to find Osiris' phallus, so she magically created one in order to conceive Horus. The question of virginity taken by a magic penis remains to be argued.
Isis Pelagia (Isis of the Sea) was only one of her many epithets. Her association with waters began with her husband, Osiris, who enjoyed wider worship earlier than Isis. He was "the god who ruled over the cycles of fertility and growth"; in the months before the Nile's inundation "Osiris was effectively dead as a fertilizing power ... and Isis mourns for her lost husband" (Naydler 70-71) It is then the tears of Isis that cause the Nile to rise, revivifying and repotentizing the Nile, the land, and Osiris. Osiris, however, is still "dead" and becomes the god of the underworld (Osiris is depicted in Egyptian art as blue or green, which symbolized a deceased person). Their son, Horus (Harpocrates in Latin), takes the divine throne, and is the "protector of life, the guarantor or order and harmony on earth." (71) What emerges in this trinity is that Isis straddles the bridge between life an death, facilitates passage from one realm to another, heals the sick, brings bounty to the living, and is mother to the high ruler.
In Greco-Roman times, Isis became associated with the Mediterranean as well. She is associated with Sirius, the Dog-Star, which nagivators used.
All that said, I would argue that it is Osiris, not Isis, that is chthonic.
I think that Magna Mater may be as much of a factor in this equation as can or any other deity fit with similar qualities (life-giving, nurturing, etc.). Magna Mater is really just a Romanized form of the Phrygian Cybele and the Greek Rhea. In the same way that Magna Mater has changed, so have other gods with migrational skills. But if you're thinking about the neolithic "mother goddess", well, that's out of my histrocial period
I'm not sure what I think about a Roman desire for a "Holy Mother." Rather, I think of it as their perceived need for a "personal savior". Women may have been particularly attracted to Isis because they could fully participate in the religion and attain the highest offices.
Religious ideas and practices change through time and locale, especially when attempting to migrate into a "foreign" environment. Religions have to translate "religious elements from a language and culture of origin into a new language and culture and also adapting the religion to the new environment." (Bowen 150) By maintaining a set of core beliefs, while modifying peripheral beliefs or behaviors (mainly by syncretism), they were able to make the new religion less foreign while maintaining its exotic appeal.
I know very little about Mithraism, so I can't comment on any lines that can be drawn between the two. But the spread of the cult through the armies needs to be carefully considered. Isis' worship was disseminated through the military, trade, friends, families, slaves, and probably any type of social group you can think of. The military was critical in terms of sheer geography, but aside from that, I think they were neither a less nor a more effective means of transmission.
Sources:
Bowen, John R. Religions in Practice: An Approach to the Anthropology of Religion. Boston: Pearson Education, 2005.
Naydler, Jeremy. Temple of the Cosmos: The Ancient Egyptian Experience of the Sacred. Rochester: Inner Traditions Int'l, 1996.