I have a theory about a scabbard design that could work, but it would have to be specifically designed for that purpose, and it would not cover the blade, just the point.A sword cannot be drawn from the back. You can store it on your back, but you can't use it until you move it back to your hip. (Look up Metatron's YouTube channel, I think he's the one who tested it out.
Maybe Beren doesn’t have each at the same time, he might lose Dagmor and use Narsil after taking it from Curufin.A hand-and-a-half sword (which is what is sometimes meant by a bastard sword) can be worn on the hip without a problem, I just don't think we need to have Beren running around with two swords hanging off his hip.
So how would Narsil get to the Eresseans? It was forged by Telchar. And how much importance can be placed on it that would be an heirloom if the Eresseans had it and did not partake in the deeds of those days?You're right, that won't work.
So, gifted to the Lords of Andunie by the Eresseans?
So it shouldn’t be the Eresseans who give it.The Elves of the Houses of Finarfin and Fingolfin had closer relations with the Edain than did the Feanorians and Sindar, so it most likely was passed to Numenor by Noldor ultimately from Fingolfin or Finrod's realms. Finrod had closer relations with the Dwarves than did the other non-Feanorian princes.
But. Narsil could have passed through many hands, like the Dragon-Helm, so there are lots of options for how those particular Noldor got it. Feanorian works got into Thingol's hoard, and all the Noldorin houses/realms probably traded with each other at least to some extent. The Sindar might have traded Dwarven things to the Noldor on occasion.
What sort of story do we want to tell with Narsil?
If it is a gift from Eresseans, why a sword? Numenor was a peaceful place. Was it given at the time when the Numenoreans had started helping the Elves in Middle-earth against Sauron, and the Eresseans wanted to pitch in somehow to help their kin?
If there isn't a good reason for Eresseans to give a weapon (instead of a peaceful gift) then it would need to come to Numenor with the Edain settlers, somehow. But in that case it could have been passed down from Hithlum or Dorthonion.