My thoughts on the subject (but first a quick aside about many of the 50th level spells):
50th Level Spells: I feel that most of them are cop-outs. While it is kind of neat, and maybe appropriate, for some of them to work as they do (allowing the use of the lower level spells on the list without cost for the duration), I would rather a cooler epic-level type spell. For example: Fire Mastery (50th level, Fire Law), the caster is able to use any of the lower-level spells on this list and is master of all fires within (set specific range, like a few miles). This allows them to control (move, grow, shrink/douse, etc...) the fires limited only by their imagination, and those imposed by the GM. The exact effects are up to the GM. This is only a very quick example just put together in a few minutes, so could/would use some refinement, but I hope it illustrates my meaning. It costs a lot to get to a 50th level spell, most of that cost in empty spell slots, so the spell should be really, really cool and strategic* as opposed to tactical in scale.
*What I mean by this, is that most magic in RPGS is geared for adventurers. I get it, it is who the games are focused around, but I have come to prefer a whole-world approach to how things are done. Meaning, you make the world and then fit the PCs and what they can and cannot do into that. So strategic means, in this instance, to effect more than on a personal/small scale; to affect a much larger scale such as city-wide, large battlefield-wide, etc... not just a few individuals or a few dozen square yards of area, which is tactical.
Channeling, Essence, Mentalism, & Arcane: Arcane is the term used to refer to the strongest concentrations of essaence; the ley-lines, nodes and stuff. What bleeds off of those inot the world around is the base essence that magicians and other use, including mentalism (though the way they go about using it differs greatly from magicians and other "essence-users." Channelers are granted the ability (to a much lesser degree) to use the essence by their respective dieties who are currently the some of very few, generally incredibly powerful beings that can tap the raw arcane (where they get their power) with much less/little/no difficulty - hence why they are the gods and we are mere mortals. Those mortals who do gain access to the arcane forces (which can be in a number of ways including level/experience, quirk of birth, location, what have you) do so at risk, for the sheer power of tapping the arcane forces is like trying to swim against a rip current: very difficult and probably a bit stupid. It is much safer (but still not perfectly so) to just use the regular waves like everyone else; they are more steady and you can better predict how they are going to flow. (Damn, I can't believe that waves/surfing are such a good analogy for magic. Now to create Bodie, the totally-gnarly surfing sorcerer.) Now, does that have a huge mechanical effect on things? Probably not, it is more of a flavor thing. Though it could be used to explain why "regular" spells come in lists, but you could have arcane magic "spells" as seperate entities, like spells in HARP are handled - only with much higher costs and greater effects. Arcane magic is so raw and powerful that only the gods and some other powerful beings and very, very few mortals tap into it, so no formalized training/lists. (Though I have to admit I like the Arcanist, Wizard and others from the Arcane Companion... dilemas, dilemas.)