While I admire all this, I think that it is handling the situation Rick started with incorrectly. As far as I can tell, he was aiming at people who were either slightly beyond beginners or people who were musically sophisticated but were somehow blocked over time from learning music theory. I think it is better to try and get at the blocks and misconceptions than to try and inundate people with theory, however simplified.
For the second group, what would be useful are things that people already know that could be enhanced by theory. For instance, on the guitar, there are these diminished chords that hold their pattern up the neck, repeating. How does that work? That would get people into a huge array of theory. And so on.
For the first group, among the big blocks are misconceptions that are obstacles that sophisticated musicians would be stunned to hear about. I speak from bitter experience, and talking to others. Among them:
On the guitar, certain chords are difficult to play, so they are assumed to be complicated -- early on, for example, I always assumed that the F chord was some bizarrely complicated chord, much more esoteric than a G chord. An A chord must be really simple because it is just three strings beside each other. Later I thought the an "augmented" chord, God it must be insanely complex because it looks complex.
Internalizing the half-step, whole step structure is often a disaster for learners. It takes people a very long time to connect going up a key on a piano or a fret on an guitar to that. It ought to be obvious, the most obvious thing, but isn't, partly because of the visual confusion (on the piano, for ease) between going from certain "main letter notes" like B to C or E to F which are right beside each other, and jumping up to something with a flat or a sharp, which must be different, smaller or maybe bigger.
On a guitar, this is even more confusing because the strings are different, and the jump from one string to another in scales seems random. It takes a long time to focus on the pattern of the scale over the range of strings, and not the pattern of the 6 strings themselves.
What else can I think of off the top of my head. People think that a minor chord must have a flat or a sharp in it, because it has the word "minor" in it and sounds flat or sharp; when it is really just the dropping of the 3rd of the chord one half step (which is of course sometimes the equivalent).
I am sure there are lots of other weird things that block people's understanding of chords.
Here's another big obstacle, the ridiculous chord notation system. Hands up all those who know the difference between a G9, a Gadd9, and a Gsus2?
I could go on.....
yours, Peter T.