Yeah, Townsend was way too young to play Aragorn. I remember the leaked image of Townsend in costume as Aragorn (I think it was just a makeup test), and he looked all wrong for the part. Sucks that he was let go, but I think they made the right decision.
But I don't have any problems with Townsend, as an actor, and I really like the idea of him as Bard. Moreso than David Tenant, a.k.a. the Walking Beanpole.
I'm not terribly worried about Saiorse Ronan's character. It's likely Itaril will just be a minor role with no major impact on the story. I'm guessing she'll have a couple small scenes when Thorin and Company are in Mirkwood, and she'll probably be at the Battle of Five Armies, too.
Plus there were a lot of random minor characters in LOTR who weren't actually in the books, and none of them really made a major impact on the story. Well, except for Lurtz.