Hi Mark

Nicely put, though I have a couple of clarifications in reply to yourself and Nick's response to offer.

You said...

Optional Double planet clause/exclusion:

Double planets allowed: Original proposal based on Barycenter.
Double planets excluded: Only the most massive object in a group of objects sharing an orbit around the Sun will be classified as a planet, the rest will be considered moons or satellites.

There is a mass ratio above which Lagrange Point objects become unstable and the system is more like twin objects competing in the orbit. I think the ratio is about 0.0435 or thereabouts. Below that and the smaller mass happily remains a Trojan, but above it and the two objects significantly compete for dominance. Ceres is in this situation with Pallas, Juno and Vesta.

Another at the top end of the scale is "brown dwarfs". Some see a natural transition mass at the deuterium burning mass limit of 13 Jupiter masses, which is not coincidentally the point where the discovered 'exoplanets' decline rapidly in numbers. For many people that's a sign of a physical distinction between the two classes.

Finally the recent thermal and geophysical modelling of Ceres tells us that it is ~25% ice, and thus it's more like Triton and Pluto than Vesta or Mars.

Adam