There is a bit of conflicting information about this herb. One version of the Lay of Leithian had Huan bring Athelas to Luthien in order to heal Beren. However, it's stated in LOTR that the Numenoreans brought it to Middle-earth. These accounts are not necessarily mutually exclusive, if the plant originally grew only in Beleriand.
One source says only the Rangers of the North remembered its true healing properties, while another states Elves also knew about them. Which one do you think is more accurate? Aragorn most likely learned herb-lore and healing in Rivendell, but he probably also learned many things while he was out in the wild. So either theory is possible.
I think as the Dunedain are descendants of the Elves they most likely learn about it from them. I always think of the stories as legends to told be different people and perspectives but in the same universe, if that makes sense.
That makes perfect sense. If you look at the Greek and Norse myths, there are variations on the same stories, and the people didn't mind that at all. "Canon" seems to be a modern concept.
I would say the Elves knew about the full potential of the use of Athelas. I doubt that knowledge would be lost to them.