This is another of those threads which is so vast and erudite that I know I will have to take a deep breath before hitting "post" at the end of my ramblings.
First is to point out is to point out the silmilarity between the journeys in The Hobbit and The Lord of the Rings (cf Paul Kocher ?) Ie Leave Shire at instigation of Gandalf, arrive at Rivendell where Elrond dispenses wisdom, dangerous crossing of mountains , part with Gandalf, cross river into magical Elvish realm, which is left by river... ultimate destination a perilous mountain... of course there are differences to but in some respects the Lotr is a "Grown up" version of the Hobbit. Tolkien isn't afraid to use the same elements and let them play out in diffferent ways (several stories in the silmarillion have similar elements). However it was the separation of the threads in TTT which stalled my first attempt at reading LOTR when I was about 10 (Father Christmas had noticed I had enjoyed the Hobbit
). By the end of the Christmas Holidays I was struggling across the Ephel Duath with Frodo and Sam and by the time I rejoined Gandalf and Pippin I had forgotten what they were up to and they went back on the shelf for a couple of years when I skimmed book 5 ( despite Faramir it remains my least favourite and least read part apart from Bombadil). I wonder if the complete separation of the threads is to stop it being quite so obvious that the main thrust of the plot has relatively little action? In the Radio version where they cut back and forth between the two this is alleviated partly by exploring the psychology of the 3 main protagonists whose mental journey is so much more interesting than the physical journey, and of course in the "real time" version Faramir becomes a muchstronger link between the plotlines. I must reread the relevant parts of HoME.
As for Christopher as audience, I am sure we owe him a fair bit - including the original map (if memory serves correctly). Even as a small boy he seems to have a memory to detail that proved complementary to his father's creative imagination. I really much get "The Letters" - that quote is quite touching and I wonder which Hobbit? Merry maybe.. It also ties in with my theory that the LOTR is the grown up version of the Hobbit...
As for the scouring of the Shire, I understand why some feel it an anticlimax , especially if you have been caught up with all the great deeds and great people- the wondrous elves and the noble men, but I think if you lose the scouring of the shire you lose the "point" of the whole thing. It is the Hobbits, that Tolkien identifies himself with and I sense that he expects us to too - much as we might fancy ourselves as Aragorn or Galadriel, Faramir or Eowyn - and we cannot live on the heights for long. The hobbits have to go home and the reader has to get back down to earth. Again it reprises "The Hobbit". Bilbo returns to find his home in the narrower sense overrun ( albeit non violently) and he has some bother before it is restored to him. Frodo and Co return to find their home overrun.. I think this shows that we cannot insulate ourselves in our own little world and keep the outside out forever (I am sure I have said this elsewhere but cannot remember which thread ) - nor can we leave and return to find it unchanged. Frodo's words about going to save the Shire, and it having been saved but not for him are perhaps the most moving and significant for me and I think that the character of Frodo lost out most of all in the movie version. I know there are cinematic reasons why they simplified the story but I find the book (and Radio) Frodo, facing middle age and making a choices to go (rather than running away all the time) so much more moving. Especially when the other hobbits are able to find a degree of fulfilment in Middle Earth. Enough rambling