I assume you are familiar with the isochron page at talkorigins.org.
There is a test for two-factor mixing discussed there, which isn't 100% accurate. Of course we can be sure that few isochrons are mixing isochrons because, were many isochrons mixing isochrons, about half of them would have negative slope.
Some creationist came up with a three-factor mixing scenario which would be nearly impossible to detect. But it required one of the factors to have none of one of the isotopes, a very unlikely occurrence. I don't have a pointer to that one.
Are you going to cover discordia? (Whoopsie, I see you did, but didn't call it that). Although sample selection and preparation methods, along with analyzing incredibly tiny samples such as a spot 10 microns diameter and 1 micron deep from a zircon, have increased the number of concordant dates dramatically, there's still room for discordia. E.g. Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago which, as of the last time I looked, covers the oldest minerals found on Earth and has an incredibly clear discordia line:
Hum. On reflection it looks as if you didn't cover discordia, you did a Pb-Pb isochron section. I suppose you don't want to go into the issue of anchoring the Pb-Pb isochron, which is fundamentally different from other isochrons, and the role of the Canyon Diablo meteorite in that anchoring?
Does the math of the Pb-Pb isochron require that we know the initial Pb-Pb ratio of the solar system?
Yup. Or something equivalent. The X and Y intercepts of the Pb-Pb isochron are meaningless. "Standard" isochrons start at the initial parent / nonradiogenic daughter point on the Y axis and can be considered to start there or be "anchored" there, since that point doesn't change over time. The Pb-Pb isochron is anchored at a point that is initially unknown and cannot be extracted from samples of one rock or lava flow or whatever. That point is the primordial lead ratios of whatever the sample came from, and the isochron forms a straight line only if the items sampled align with the line between that point and modern lead ratios (which are difficult to sample averaged across the entire Earth). Dalrymple devotes his entire last chapter to it, so it's obviously difficult to condense to a portion of a message.
The Canyon Diablo meteorite (AKA Meteor Crater AKA Barringer Crater) has so little uranium and thorium in it today that it cannot have had any significant amount of uranium and thorium 4.5 Bya, and therefore its lead ratios today are primordial for the Solar system and are the anchor point for that Pb-Pb isochron. Since then similar meteorites have been found. The achievement of Patterson's classic paper of 1956, Age of meteorites and the earth, which established the currently accepted age of the Earth, was to produce an independent and believable tie between meteoric lead and terrestrial lead.
The math of how the points come to lie along the Pb-Pb isochron is considerably more complex than the "standard" isochron, because the points "move" on the graph over time in a significantly more complex manner. Points representing individual samples on a "standard" isochorn "move" over time in straight lines, as illustrated at http://www.talkorigins.org/...n-dating/AnimatedIsochron.html. Points on the Pb-Pb diagram "move" along growth curves, two of which are shown in the figure above.
It might be nice to include U-Th disequilibrium dating of coral in the calibration section. Tree rings only get you to about 12,400 years BP. I happen to think that U-Th disequilibrium dating is a very clever method. It's been used to date corals, bones (exposed to groundwater containing U), a plaster stalactite in the Siloam tunnel in Jerusalem, and more. But you may not want to go into detail of how it works. Wikipedia has reasonably good and short article.