If I remember well, in Tome of Magic there is a class called True Namer, that bases his spellcasting abilities on learning the true names of things in the universe in order to bow them to her will..At the moment I don't remember how this system works at all, but you may consider taking a look at it, to me it seems to be fitting the theme! One of the point of the class, for example, is learning your own true name (Eragon himself comes to mind).

Anyway Dragon Rider should definitely be a prestige class with quite high prerequisite, while magic as a skill could work. In the end, everyone has the potential to use magic in the Inheritance world, Roran himself start to practice with the stone ("stenr reisa", am I correct?:) ) and after a while he was able to use a little bit of magic if I do remember well! And he surely didn't do a dip in a spellcasting class:)

In my humble opinion, to be honest, every spellcasting class (apart from a base one) should be a prc, reflecting how the spellcaster decide to specialize himself (summoning, spirit bounding and that kind of things). Lastly, about herbalism, someone some years ago started a thread (unfinished) called The Laboratory: Alchemy and Herbalism, when he was proposing a new subsystem..with a little bit of google fu you can easily find it. In my opinion this could be another good way to introduce Angela's herbalism.:)