I find this letter very rich and very insightful based on 3 of Tolkien's comments here.
The first Voronwë you have quoted where Tolkien states what he feels the real theme is:
about something much more permanent and difficult: Death and Immortality: the mystery of the love of the world in the hearts of a race 'doomed' to leave and seemingly lose it; the anguish in the hearts of a race 'doomed' not to leave it, until its whole evil-aroused story is complete.
So the story is about the view of the Elves on Middle Earth, whose destinies are inter-connected to it until the final battle, and men, who are destined to live on it for a short period and then die, leaving it, seemingly loosing it forever. I think this brings about a change in views in terms of what Elvish immortality means, which is to last until the Final Battle and after that we are not sure, though the Elves I believe think that Arda will be remade unmarred and immortal man and them will exist on it.
It also brings up the notion of the destiny of man, using the story of Aragorn and Arwen. Aragorn knows that as a mortal, that his day will come where he will lay down the great lifespand that had been granted to him. His last interaction with Arwen shows that he has a hope, that beyond this world they will meet again. Arwen for perhaps the first time has pity for the fate of mankind, and with bitterness accepts her fate. I guess for her in a sense, if she did not understand the nature of men's belieft that the body would die but their spirit would go on to a light or brighter place and instead felt that humans died and that was the end, it would be bittersweet. That combined with the fact that Arwen was not "weary of her days" yet was perhaps hard to handle; the fact that she could not choose her own time as Aragorn was giving, but that death is imposed on us.
I don't buy into that thought that Arwen did not know of the belief of men on what happens after death, for Aragorn reminds her that "Behold! we are not bound for ever to the circles of the world, and beyond them is more than memory. Farewell!" Aragorn had a very strong belief in what happened after death to men and women, yet it seems that for Arwen, raised with the beliefs of the Eldar, the reality of that belief had not reach her yet, nor the impact of her choice. For the Eldar, they live on and things in the past are that, a memory to them and that life will continue until the ending of the world. Two very interesting perspectives, immortality until the end of Arda vs death and release from Arda to something greater, with only a short lifespan. This section of the letter had made me decide to go back into Morgoth's Ring and look at Athrabeth Finrod AH Andreth and explore Tolkien's investigation into these concepts.
Tolkien's next main point of the story comes from a quote by Elrond:
"Such is oft the course of deeds that move the wheels of the world: small hands do them because they must, while the eyes of the great are elsewhere."
I think we see this throughout the trilogy as small hands do the work because they must, i.e. Frodo, Sam, Merry, Pippin, while the eyes of the great are elsewhere. Yet it is this notion of small hands that do the work that is in fact one of the things I think most people relate to in LOTR. Most of us are very common people, yet it is the common people that rise up when called upon or when need is present, and do the common work that the great never get around to. It is the common woman or man that does his daily tasks that turn the wheels of the world (interesting choice of words by Tolkien I think). I wonder how much of this came out of his World War I experience?
Finally, Tolkien quotes one of my favorite passages from Merry, stating it is equal to the one in the last paragraph:
'the soil of the Shire is deep. Still there are things deeper and higher; and not a gaffer could tend his garden in what he calls peace, but for them.' I like the whole quote as "Dear me! We Tooks and Brandybucks, we can't live long on the heights. 'No,' said Merry. 'I can't. Not yet, at any rate. But as least, Pippin, we can see them, and honour them. It is best to love first hat you are fitted to love, I suppose: you must start somewhere and have some roots, and the soil of the Shire is deep. Still there are things deeper and higher; and not a gaffer could tend his garden in what he calls peace but for them, whether he knows about them or not.'"
I find it interesting that here we have the heirs of the Took and Brandybuck families who will be something someday in the Shire, as much as anyone can be something there, and yet they have realized that the world is a much broader and bigger place then just their home. Often I think for Merry and for Pippin this journey has been one of discovery, about themselves and about the larger world. I think Tolkien is hinting at though it is important to be well cemented where we are, we have to remember that their is an entire world out there and we need to be able to reach out and glimpse it, and be a part of it. We have to reach out beyond ourselves in order to benefit others, and to see the benefits that others bring to us. We don't need to be more than what we are, but we do need to be.