I would posit a few things. I am a Virgo, and I definitely have an intuitive hunch, if you will, that Chiron rules Virgo. But I am no authority on the matter, and I lack enough experience with the wealth of astrological data available to really argue the case.
On the contrary, I would like to present some ideas for why Mercury fits the idea of rulership of Virgo, at least as far as the traditional rulerships go.
There are a few things we need to clarify about Virgo, about earth, and about Mercury in order for these ideas to fit together. Let's begin with Virgo.
Virgo. Mutable earth. What type of earth is mutable, changeable, reconfigurable? That type of earth is clay. It can be molded to any form, and then, so long as it has not been fired, it can be remolded into another form, almost infinitely. As someone who has worked with clay as an art medium, I can tell you that there is a certain mixture of elasticity and solidity that is preferable for working with clay. Too moist, too elastic, and it won't hold shape; too dry, too solid and brittle, and it will crumble (also not holding shape). So we have the image now of Virgo as clay.
Earth is the most receptive of elements. Some would challenge this idea and say that water is more receptive; however, if you mold water into a form, a container, and then remove the container, it does not hold the shape. It again returns to fluidity (with the exception of ice or fixed water, but even that is only temporary). It has not received fully the idea that was impressed upon it by the form. Earth can be molded into a form, a container, and upon removal of the container, the earth maintains the shape. It has fully received and maintained the impression or the idea of the form. Therefore, it is more receptive. (This idea also gives validity to the idea of earth signs as more stubborn than others. Once they've received an idea, it's more work to remold them.) Also, on a molecular level, a substance in solid form (earth) has the least amount of space between atoms. The transmission of energy from one atom to another happens quickly. From thought to emotion to form quite rapidly. This would be why we see earth signs as being rather intuitive. They "just know" things or they "just act", but couldn't tell you why. They don't have the luxury of enough distance from a thought form in order to see it coming, like the kind of foresight or imagination we see in the air signs. Though with Virgo, being of a somewhat more fluid nature in a mutable modality, they can get that sort of lost-in-thought or spacey quality more likely to be observed in water signs, especially Pisces. The earth perspective is not from the position of one sending a message, but more at the one receiving the message. They're not dialing the numbers on the phone, the phone just rings and they answer. They don't decide to make the call, they can, however, choose to complete it.
Mercury. As a planet, it governs travel and change. As a substance, it is not unlike a very watery clay or slurry in that it is both metal (earthy) and liquid, defying most understandings of either substance. (It would seem this idea of defying understanding is a theme for Virgo.) Add to that the lore of Mercury, the god, who was hermaphroditic, neither male nor female, yet both. Some would say, at least anecdotally, that many men who are Virgo natives exhibit more receptive/feminine qualities and that many women in Virgo exhibit a more aggressive/masculine nature. (E.g.: the sissy-boy male Virgo or the strong-willed female Virgo who often gets labeled as a *****).
Taking these ideas together, we have to ask ourselves, what sort of change or travel occurs with earth? Transmutation. While it is not the transmission-of-thought kind of travel that we see with airy Gemini, it is the transmission of energy into physical form. That would certainly be a good quality for anyone hoping to cause healing. It is the movement of emotion (the prefix e- meaning forward, so e-motion is forward movement) outward into physical expression. It's the delivery of the message. They hear the phone ringing, and they answer, thus completing the transmission.
Virgo does have a dual nature. I would see her as both mother and daughter, as someone posted earlier. I know astrology doesn't really relate to Christian ideas, necessarily, but if we can use the mythology of the Greeks and Romans, why not the mythology of the Christians? In this way, I see Virgo as the Virgin Mary. Both virgin and mother. Pure and caring. An expression of love for love's sake. It would also make some sense, as some have undoubtedly heard here and there of the idea that Jesus was a Pisces, correlating with transcendence and martyrdom. Are they two sides of the same coin as are Virgo and Pisces?
But I digress. I hope I have given a clearer picture as to how Mercury could rule Virgo. I absolutely believe in the dualistic expression of every idea, so the traditional planetary rulers being split between one masculine sign and one feminine sign makes sense. There is also the idea of the outer planets, Uranus, Neptune, and Pluto - and Chiron - as being
rulers of a collective consciousness and outside of our ego or personal control. This idea has me thinking more broadly in terms of duality. What if there are co-rulers to each sign? So it's not that Pluto has supplanted Mars as ruler of Scorpio, but that one is of a personal nature and one is of a more collective nature? Mercury could be the "personal" ruler of Virgo, or the ruler of those parts which are incorporated into ego identity, and Chiron could be the "collective" ruler of Virgo, or the ruler of those parts which affect society, or things that cannot be fully kept under egoic control because they are not our to control. If that be the case, then it's really a moot point to argue rulership because each sign would have two, in which case, just think of all the possibilities for planets that are to be discovered or of which asteroids/planetoids might be assigned a rulership. It's such an expansive thought, one that has me connected to my inner Pisces.
Namaste to all.