*shrugs* I've always assumed that Frodo wrote down what Sam told him, though whether it's because he can't remember or won't remember on his own is up to interpretation. ;) And for the return to the Shire and the Grey Havens, well, I think Frodo wrote a lot of the scouring part (which is why his role in everything is understated, and why him hiding his illnesses is acknowledged), and then Sam had to write the Grey Havens, obviously because Frodo was gone by then. :p

What conversations did they have between the time that Frodo admitted his intentions and they actually reached the harbor?

I'm going to try not to think about that, for fear of getting the hamsters into a plotting frenzy. 0:)