Feanor and re-embodyment

[Thingol1000]

I was wondering about Feanor and his time in the halls of Mandos, so I heeded the board wardens call to search and this is what I got;
"and the fact that Feanor will never be reincarnated before the Second changing of the World and Saeros, who was supposed to spend a lot of time in the Halls"

Where did this statement come from? (Text, not the person) Did the Valor decree this becuase he instigated the exile (or the first bloodshed elf upon elf)
Who and why did Saeros spend so much time in the halls of Mandos?)
Thanks

[Saurreg]

Saeros was a kinsman of Daeron, greatest of the Sindarin lore-masters of old and chief advisor to Thingol of Doriath. He tried to take the life of Turin Turambar in an ambush but was overpowered by the human. Turin then tore of the clothes of Saeros so that he was naked and then pursued him with a sword in hand. As Saeros reached a cliff with Turin hot on his heels, he attempted to jump across to the other side but failed and plummeted to his death.

[piosenniel]

Saeros:

"So he [Saeros] ended his life in Doriath, and long would Mandos hold him."

from: Unfinished Tales - 'Narn I Hin Hurin'.

Feanor:

"But not until the End, when Fëanor shall return who perished ere the Sun was made, and sits now in the Halls of Awaiting and comes no more among his kin; not until the Sun passes and the Moon falls, shall it be known of what substance they [the Silmarils] were made."

from: The Silmarillion - 'Of the Silmarils and the Unrest of the Noldor'