A lot of minute tolkien lore before or after the hobbit and LOTR wasn't published while Tolkien was alive (silmarillion being an exception). He often changed stuff during the writing process. Christopher (his son) compiled his notes into semblances of finished work but it is debatable whether things would have been written as they are for things like children of hurin and other smaller works. So it's official in that it was published but may not have remained "canon" if Tolkien was alive.

The only other half-elf who died of natural causes after choosing mortality was Elros, Elronds brother and Aragorns great(× a few) grandfather. He lived to 500.

As handed down through generations of river folk songs.

Modern day Riverdance.