There are no classes (I don't use that term to describe any of the types of Vocational Kits). I do use the word Types, and the four types are simply there for organizational purposes mostly.
Arcanist is the type of Vocational Kit that focuses on magic. The Envoy Vocational Kit is an Arcanist Kit, the Mystic is an Arcanist Kit, etc....
Mage = a character focused on a single School of Magic. This could be an Envoy/Envoy, an Envoy/Champion, a Warlock/Cabalist, a Wizard/Wizard, etc....
Magus = a charact who has access to two Schools of Magic. For example, one of the Pregens is a Magician/Bard (His Schools of Magic are Magery and Wizardry). Another Example would be a Mystic/Champion (Mysticism and DIvinism). (side note: in a game I used to work on for another company - this type of caster would have been called a Hybrid)
I do see that I can better define Mage in two locations to make the difference easier.
Yes, in some games a Half-Elf would be considered a Hybrid.
You said:
4. Mixed-Focus characters draw from two primary focuses combining them into a unique focus.
That is basically the definition of a Hybrid. Two things mixed to become one.
Now, in Novus, I used the terms Casters, Fighters, Adepts, and Hybrids..... I could always change Arcanist to Caster