[Legolas]

Morgoth's Ring:

But the sons of Men die indeed, and leave the World; wherefore they are called the Guests, or the Strangers. Death is their fate, the gift of Ilúvatar unto them, which as Time wears even the Powers shall envy. But Melkor hath cast his shadow upon it, and confounded it with darkness, and brought forth evil out of good, and fear out of hope. Yet it is said that they will join in the Second Music of the Ainur, whereas Ilúvatar has not revealed what he purposes for Elves and Valar after the World's end; and Melkor has not discovered it.

This quote from The Silmarillion should resolve both points of confusion - shows Beren did go to Mandos, as did all men, and describes Mandos' power over these spirits:

Therefore he {Mandos} summoned Beren, and even as Lúthien had spoken in the hour of his death they met again beyond the Western Sea. But Mandos had no power to withhold the spirits of Men that were dead within the confines of the world, after their time of waiting; nor could he change the fates of the Children of Ilúvatar. He went therefore to Manwë, Lord of the Valar, who governed the world under the hand of Ilúvatar; and Manwë sought counsel in his inmost thought, where the will of Ilúvatar was revealed.