I want to take pictures of a dentist's work inside the patient's mouth, while looking right over the shoulder of the dentist.
What configuration of lenses, teleconverters, and macro or zoom characteristics will allow a photographer to take a picture of a field as small as a few millimeters squared, with absolute definite clarity?
At the same time, a respectable distance from object to lens must be maintained to protect the patient or the equipment from being contaminated...probably 24 inches. This distance will allow me to work over the shoulder of the dental surgeon without being in his way.
I'm certain that a snap on light diffuser will be required, and possibly a 2x converter. What lens combination will do the job?

Close-up work from a distance requires long focal lengths. The longest macro focusing lens I'm familiar with is 300mm, and I don't think it would be enough for what you propose. Another possible solution might be something on the order of a 500mm lens with macro extension(s).
I would think the most practical solution for your situation would be a fiber optic camera, such as those used for inspecting the insides of rifle bores and small tubing. Since the electronics are all in the camera, the fiber optic part can be sterilized separately. This would require the end of the cable being very close, such as inside the patient's mouth.
You didn't say if the pictures were going to be still shots or video, but the principle is the same, though for still shots, you could have the dentist move his tools and hands out of the way.

Macrophotography generally presumes a 1:1 magnification ratio, which would project a full size image onto the image sensor. It at least would require a 1:2 magnification ratio, which would project a half-size image.

What matters is how much of a person's mouth you want to appear in each frame. My guess is that, at most, you'd want a single tooth accompanied by the image of half a tooth either side. But for general shooting, you'd want 3 or 4 teeth, possibly including one or more dental instruments, and even one or more of the dentist's hands.

Historically, this type of photography was accomplished with a 35mm film SLR and a 100mm lens, but it would be operated by the dentist, not by a third party observer.

I'd say that you'd need a high resolution image sensor, at least 20MP for an APS-C body, or 40MP for a 'Full Frame' body, in order to capture the detail you want. You'd also need a long macro lens. Sigma has a 150mm macro lens with a magnification ration of 1:1 and a minimum focus distance of 15 inches. It's also stabilized which will help when shooting hand-held. (The longest lenses available that are capable of 1:1 magnification are Tamron's and Sigma's 180mm lenses, but they're not stabilized.) You should also get a ring light or ring flash to supplement the available light, or you'll need to use large apertures which will limit the depth-of-field you'll get. If you need to get farther away, you could use a 1.4X teleconverter, but I would avoid using a 2X teleconverter if you want sharp photos. I'd also avoid using close-up lenses for the same reason.

All this gear is available for rent at LensRentals.com, so you can rent it all to see how it works for you.