[quote=@Genkai]
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.
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I had imagined as a sort of magic-fueled pseudomortality: people are biologically immortal, that is to say unless reaped by disease, war, or something else, the will live forever.
So death is still possible, just unlikely without some existential threat at work.
Thus, you'd have extremely old civilizations where internal strife over resources and encroaching dangers of the magic wilds outside keep the populations low.

As for magic.... 
I know I separated them with "OR", but you could actually combine both ideas.
For example, magic is the power source, the magic language is the interface, and the borrowing of aspects from reality is what magic lets you do.
For the sake of consistency, it'd probably be easier to stick with the ambient power source idea. The rest of it can be whatever.
Using plants as a focus for various kinds of magic could be interesting, but would be supplemental I think. Perhaps such ideas are enshrined in a particular tradition of magic?

I don't really have a preference, since I've written within the contexts of these different expressions of magic before. Whatever sounds fun to you is what we'll go with.