one other point re: lake Evendim and others being raised in Rivendell.
I think [meaning this is my speculation] the Northern Dunedain had made it policy to:
1] stay more or less in hiding as a community, to avoid being targeted as Arthedain and Cardolan were by Sauron, who as we hear in Rivendell has been searching to see if any hear of Isildur still exists.
Any Heir of Isildur would be a triple threat: 1 - he might have or [more likely] know of the whereabouts of the Ring,
2 - would be seen as a personal threat by Sauron as the heir and inheritor of the sword that defeated him in the Last Alliance.
3 - He could potentially [ in Sauron's mind I imagine] rally both Gondor and Rivendell and even possibly Lorien against him.
Michael Martinez has done some very interesting research into Arnor you can check out:
http://www.suite101.com/article.cfm/tolkien/73611
and more pertinently [in terms of the northern Rangers]:
http://www.suite101.com/article.cfm/tolkien/64660
enjoy.