What I'm trying to establish is where theory starts and possibly ends.
I'd say that music theory begins with Rhythm, Melody, and Harmony, which are the three basic components for every song that is composed. When a person starts to learn music theory, that's where they start. Even if someone starts playing an instrument (without learning theory), this is where they start as well! They learn theory into practice without even knowing it.
It ends .... nowhere. Basically people "invent new music" everyday, especially nowadays with all the technology they can use. So, there can be theory that explains that music.
So, learning the C major is scale I'd say is part of music theory; one of the very basics you'd need to know, but still part of it.
Does history of performing artists come into theory
I'd say yes and no. I find it important to know the history of the music, so that you can understand where the composers/performers came from. Why did Bach compose the way he did? Why was Stravinsky the way he was? etc.
The historical background of these composers played an important role on the way they composed and performed.
Is there a nice simple formulation of what constitutes music theory?
Simply, music theory is the way of explaining music; the way to understand music; the possibilities, the practices and generally what is going on on a song. Basically the grammatical rules of the written language of music. It explains what is going on when we listen to music.
Wikipedia provides a nice explanation from the The Oxford Companion to Music
The first is what is otherwise called 'rudiments', currently taught as the elements of notation, of key signatures, of time signatures, of rhythmic notation, and so on. [...] The second is the study of writings about music from ancient times onwards. [...] The third is an area of current musicological study that seeks to define processes and general principles in music — a sphere of research that can be distinguished from analysis in that it takes as its starting-point not the individual work or performance but the fundamental materials from which it is built.
The Oxford Companion to Music