What about.
Rumil wrote the Ainulindalë. -->Pengolodh rewrote it based in Rumil and added things.-->Pengolodh wrote Valaquenta, Quenta Silmarillion, etc.
This could have been in Gondolin, in the Second Age or in Eressëa after his return.
The Elves visited Númenor--> made transmission of the "tales".--> Men passing many years (Sauron in Númenor), filter the stories.--> The wrote or oral transmission passed to Arnor, preserved in Imladris.
Elrond never talked with any Valar. Glorfindel only to help.
Bilbo translate-->Findegil's copy of the Thain Book-->Tolkien translate--> We read.
Greetings.
PS: Hello my colleagues of TFTE. Here I am. Very, very busy, years older, without time to work, but with the project in mind..... in the future?