Gosh, I'm sure it won't matter when the IC is up and running but it does make plotting a little more tough. >__>'

I think the tweaks you made were fine for both sections, though I am a little unclear (maybe it's just the phrasing?) about "However, people began to live longer and longer despite it. Under the strain of myriad threats, a stagnant populace of nigh immortals, civilized societies became impossible to maintain." So with magic killing off people, there's some who are immortal instead of diseased? Or do you mean that because of sickness, it's impossible to have long standing lines of family/populations? Maybe the immortality thing was somewhat sudden but then somehow contradicted or discredited at the same time but I may need more 'filling in' with your train of there.

Alright so magic...

You had suggested two possible routes to take. I think the first would be the easiest to explain and implement. Different conduits if you will, like runes or books or flutes or rings, whatever. Each item or design would provide a user with a certain kind of magic (if we go the traditional route with four elements or something). Or if we do stick to the idea that magic is everywhere, it could just be an energy source without an affiliation to elements but maybe to just power/amount? Or weather? If it's sunny, it would be hot/fiery. If it's windy, the magic would be windy. It's somewhat like your second idea?

I do like the second idea since it does take into account the location and terrain and could make combat scenes very interesting but it might get out of hand. At the same time, if it's something you're really feeling, I don't mind going along with it. ^^

This is another crazy (read: random) thought but if we wanted to think many streams between Rathe and Earth are maybe tree related...we could have people fighting/using magic with small tree branches or something. Maybe certain trees have certain powers or abilities? Maybe if they flick it at someone, the magic would 'shake' from the leaves or branches and be directed at the target? Maybe some trees are healing, some are more for combat and after heavy use, they die? Again, just a random thought, I'm trying to add to some ideas since you added a few of your own and I don't wanna be a bum haha.