Actually, it isn't that hard to explain so it makes sense.
Who taught them? Several possibilities. While I agree that Alachia wouldn't repeat the mistakes of the past (she makes all new ones), that doesn't mean that the knowledge wasn't passed on. Remember, before the CFD plot started up, we had Artifacts from the Earthdawn era 'waking up'. At least one of them has been confirmed to be an artifact with rituals that warped some of the forests in Tir Tairngir. The people who created the Ritual of Thorns knew that magic was cyclical, having had warning of the Horrors from the previous age of magic (see: Therans), so they may well have created and hidden instructions on the ritual, so that future elves would know how to protect themselves from the Horrors.
While the major Immortal Elves likely aren't involved in this, there are, canonically, more than just the handful we normally talk about in conjunction with the Tirs. Not many, but upwards of 20, IIRC. We have no way of knowing if one of them may not have become Corrupted, or sought the Ritual of Thorns for noble reasons to protect the elven tribes.
While it is unlikely in the extreme, remember that there are quite a few dragons in that area, and dragons can be bastards. It wouldn't surprise me if the Ritual of Thorns was used as a way to try and combat the Diablo trees, and it has spiraled out of control (like magic wants to do).
But honestly? There's another scenario that makes much more sense. Amazonia is a very magical country, with a LOT of their version of shamans. Who do shamans like to talk to? Spirits. And spirits may know, or have access to the knowledge, how to modify and perform the Ritual. We've already had one major plot involving spirits and plants, as well, in that same area. If one of the shadow spirits from the Ghost Cartels plot got loose, well, that would explain a lot, no?
As for the power levels, remember that magic levels are higher now than they were during the Ghost Dance. And while the knowledge for the Ritual of Thorns is higher, the power required to change a single tribe is far less. Instead of an 'all at once' thing, I'd say 'village by village' is more likely. Both because the rituals wouldn't send out shockwaves to let everyone know what's up, and because it would require less sacrifice, maybe even none, depending on the group size, and how willing the spirits were in helping out.