Bilbo: an unreliable narrator?

[Mithadan]

where did Bilbo stop being the narrator?

Yes, this may be the point! In the recent A Question of Tone thread http://forum.barrowdowns.com/showthread.php?t=19709, this is the argument I was reaching, though I failed to actually follow up.

Tolkien dedicated a lot of text in LoTR to his role as the "translator," going so far as including in the appendices a note on translation where he compares the "original" languages. As Galin mentions, the early forward included more on his role as translator. But what was he translating? He makes it clear that he was translating a copy of the Red Book and its affiliated books of lore. And who wrote them (in Tolkien's subcreation)?

Bilbo clearly wrote There and Back Again. He likely wrote, at least, the beginning of LoTR, which details events through the time that Frodo and his companions reached Rivendell. So an argument (wholly speculative) exists that JRRT, who was detail-oriented (to be polite), had assumed the role of the "translator," and had identified Bilbo as the original author, intentionally conformed his writing style in the opening chapters of LoTR to the more whimsical style (Bilbo's) he employed in The Hobbit. Because Frodo wrote the balance of LoTR, his tone was different.

The inclusion of giants, silly trolls that turn to stone, and Elves singing nonsense? These elements, possibly inconsistent with the otherwise consistent recitations of Middle Earth nature, could be explained as intentional. A combination of Bilbo's writing style (he was crafting a story, not a history) and Bilbo's lack of education, at least until later in his life. Hobbits, and Bilbo, were rustic.

So Tolkien included these elements in his "fairy story," originally written primarily for young readers, and retained them as part of the feigned nature of the narrative. Utter speculation, but a potential explanation.