Who wrote the genealogies and commentaries? Was it Frodo? Bilbo? Sam? In the Thain's Book on Tuckborough.net, it mentions that "... Undertowers a fifth volume of genealogies and other notes and commentary was compiled. These were preserved in the library of Undertowers for future generations along with other historical books and records." Could it possibly have been Elanor who wrote them or perhaps at least some of them?
My idea on that is that each of the 4 hobbits was responsible for supplying the initial genealogies: Merry for the Brandybucks, Pippin for the Took, Frodo for the Bagginses, and Sam for the Gamgees. Given that the Gamgee tree goes on well past the others in Frodo Gardner's line, then I would assume one of the Gamgee clan updated that one. Most likely it was Elanor, or one of her descendants, as the Red Book was given to her by Sam.
I agree with GF that there were likely contributions from all the Travellers and their families, but the one whom we know for *certain* compiled most of the "beyond the bounds" portions of what became the Appendices was Meriadoc the Magnificent.
"Since Meriadoc and Peregrin became the heads of their great amilies, and at the same time kept up their connexions with Roan and Gondor, the libraries at Bucklebury and Tuckborough contained much that did not appear in the Red Book. In Brandy Hall there were many works dealing with Eriador and the history of Rohan. Some of these were composed or begun by Meriadoc himself..."
"At Great Smials the books were of less interest to Shire-folk, though more important for larger history. None of them were written by Peregrin, but he and his successors collected many manuscripts written by scribes of Gondor: mainly copies or summaries of histories and legends relating to Elendil and his heirs. Only here in the Shire were to be found extensive materials for the history of Numentor and the arising of Sauron. It was probably at Great Smials that The Tale of Years was put together, with the assistance of the material collected by Meriadoc..." FotR, Prologue, "Note on the Shire Records"