I'm about half-way through my re-read, Merry, and you are right - although there are elements of the monomyth found throughout LotR, Tolkien isn't recreating it or constrained by it. Or even probably aware of it! But many of his characters and their journeys are archetypal because of his deep understanding of old tales. And also, I think, because of his deep held religious beliefs which gives him an understanding of every man's journey through life.
I don't think there is a world navel concept in LotR - a sort of 'World Mountain' appears in the Silmarilion as Tanquetil but it's not really presented as the centre of Middle-earth. But it's more of a psychological meeting point anyway where the divine breaks though into the mundane and where the One becomes Many i.e. where creation (and also, equally, destruction) happens.
On most Hero journeys the Hero is seeking this spot to gain a boon for mankind. This is where Frodo's quest runs counter to the usual Hero story because the boon he is seeking is the loss of something - not gaining something. He is going back to where something was created to unmake it. It's like putting the creative energy in reverse, going to an anti-World Mountain if you like - appropriate because the creator and creation (in this case) is evil.
Aragorn's Hero journey is much more conventional and what he seeks is a validation - a blessing - on his right to the kingship (thereby saving Middle-earth and renewing the land). I think this comes in the passage of the Dimholt where he passes through a sort of underworld and emerges from it to be hailed by the people he passes afterwards as the King of the Dead. It's where he finally unfurls his standard and says 'The hour is come at last'. It's a resurrection of the True King of Gondor and it's so full of old mythological beliefs it's nearly bursting. Passing through an underworld, going underground or going into a cave, or a whale (or monsters' belly) and facing Death before emerging victorious is found in every culture the world over. I know Campbell gives a lot of examples.
In their own way, Frodo and Sam make that journey through Shelob's layer as the precurser to finally entering Mordor. In fact the whole Fellowhip also pass that trial when they go through the Mines of Moria, meet the Balrog and lose Gandalf. When they emerge everything has changed and the Hobbits have to 'grow up'. But I think Moria is Gandalf's personal Underworld Journey where death is faced (invloving real death and rebirth), the Dimholt is Aragorn's and Shelob's lair is Frodo and Sam's.
I'd better stop - I'm in danger of getting carried away and pontificating
.
