I like Earnur in the spellbook though. He was not a man of Arnor, he came to their defense from Gondor.
I think so, he fits better to the spellbook, where is usual place for reinforcements.
For a hero killer, perhaps we could simply add the Sons of Elrond to the Rivendell fortress.
There fits much more Glorfindel, according to Witch king prophecy. He helped to Arnor against Angmar quite lot. He is maybe only one from the elven race, who really fits there permanently. We have no information about Elladan and Elrohir in this area.
Or Glorfindel could get a different role there, but I think that might be confusing. However, is role within Rivendell actually isn't set in stone yet because we haven't balanced that faction yet. So he might still change as well.
Yes, it can be confusing, on the other hand there could be difference from the Imladris Glorfindel. Angmar Witch king is also different and he is still Witch king.
Personally I really like "tank role" for him, especially for Imladris it will be very suitable.
Well, even in Arnor he can stay as a tank, and Arvedui can be reworked to hero killer role. Maybe it will correspond more, because he is available in the fortress, exactly like Aragorn for Gondor, so both hero killers will be in the fotress like both scouts.
Overall,
hero killer role seems be very important for each faction in the new version, where all heroes have different roles, they are cheaper, they cost no CP and mainly - AI really likes them.