After the 32 parentheses in A Long-expected Party, the number in The Shadow of the Past plunges to five. Of these one occurs in direct speech (Gandalf: 1.ii.53). Three present genealogical information, always of interest to Hobbits (all on 1.iii.42). A fifth wryly signals that Frodo had a bad feeling about the 'significant (or ominous)' approach of his fiftieth birthday (1.ii.43), the age at which 'adventure had suddenly befallen Bilbo.' Since Tolkien always uses 'befall' of evil or at least strange and unpleasant events, this explains the rather proleptic 'ominous' as well as pointing to Frodo as the author of this comment. For Bilbo did not regard his adventure as an evil, even when he came to understand that the Ring was; and Frodo, whatever he may have genuinely felt about 'adventures' before Gandalf told him about the Ring, certainly did not want the 'adventure' he got. It would be no surprise then, though it need not be so, if as narrator Frodo took his disquiet as he neared fifty as ominous.
Three is Company contains seven parenthetical statements, of which four are purely informational (1.iii.65, 68, 70, 81), two are humorous comments on Hobbits (1.iii.71, 77) and one again suggests uncertainty in Frodo's attitude towards something that made him uncomfortable (1.iii.70), namely the conversation he overhears between the Gaffer and a stranger later discovered to be one of the Black Riders.
In A Shortcut to Mushrooms one pokes fun at Sam's disappointment about missing the beer at The Golden Perch (1.iv.88) and the other at the way farmers complain about their prospects (1.iv.92).
A Conspiracy Unmasked provides five, three informational (all at 1.v.98), one showing Sam's mixed emotions about leaving the Shire (1.v.99), and one Frodo's about seeing his and Bilbo's things in the house at Crickhollow (1.v.100).
All three in The Old Forest suggest uncertainty. Merry isn't confident that it is the bonfire glade ahead of them (1.vi.111); Frodo doubts it's even possible to turn back (1.vi.113); and Frodo and Sam think the words Old Tom is singing are 'nonsense', but they aren't entirely sure (1.vi.119).
While the first parenthesis In the House of Tom Bombadil conveys details about the house itself (i.vii.124), the other three highlight Frodo's ambivalence regarding the Ring. Indeed these three seem to work together to accomplish precisely that in the scene with Bombadil and the Ring (all at 1.vii.133). When Old Tom returns it, Frodo suspects trickery '(like one who has lent a trinket to a juggler)'. Having put the Ring to the test by donning it, he is 'delighted (in a way)' and 'laugh[s] '(trying to feel pleased)'. It is as if on some level Frodo wished it were not his Ring, even though compelled to prove that it was. Bombadil's imperviousness to the effects of the Ring seems important to Frodo only in so far as it makes him doubt the Ring.
Fog on the Barrow-Downs is reminiscent of A Long-expected Party, which lacks parenthetical statements in the parts in which no one would find anything amusing. Here the scenes telling of the hobbits' capture by the Barrow-wight have no parenthetical remarks until the narrator reaches the moment when he recounts the awakening of Frodo's courage, a virtue 'hidden (often deeply it is true) in the heart of the fattest and most timid hobbit', and informs the reader that 'though [Frodo] did not know it, Bilbo (and Gandalf) thought him the best hobbit in the Shire' (both at 1.viii.140). There is a gentle humor in the humble concession of the first and the citation of Gandalf as an authority in the second, which suggests a resolution in Frodo we have not seen before, and the narrator's faith in that resolution. As such it marks a strong contrast with the uncertainty we've seen before.
Once Old Tom appears to rescue them the more broadly humorous commentary returns. just as it does in A Long-expected Party once Bilbo has let go of the Ring and left it to Frodo. The next five parenthetical comments, including one in direct speech by Bombadil (1.viii.144), are either amusing themselves or embedded in an amusing context (1.viii.142, 144, 145). Yet as the hobbits are about to return to the road, ending the passage through Faërie they had begun when they entered The Old Forest, even Bombadil makes a remark parenthetically that could be taken to express uncertainty (1.viii.147): 'Tom will give you good advice, till this day is over (after that your own luck must go with you and guide you)'. As always with Tolkien, however, what is called luck or chance is often far more. Bombadil's mention of luck here nicely balances his answer to Frodo's question upon their first meeting (1.vi.126) and thus bookends their acquaintance:
‘Did you hear me calling, Master, or was it just chance that brought you at that moment?’
Tom stirred like a man shaken out of a pleasant dream. ‘Eh, what?’ said he. ‘Did I hear you calling? Nay, I did not hear: I was busy singing. Just chance brought me then, if chance you call it. It was no plan of mine, though I was waiting for you. We heard news of you, and learned that you were wandering.'
Consider also consider that even as Old Tom tells them they must trust to their luck, Strider -- unbeknownst to the reader and the hobbits (and Bombadil?) -- is on the other side of the hedge dividing the Downlands from the road (1.x.163-64): Strider, whose role and arrival had been foreshadowed that very afternoon outside the barrow in Bombadil's conjuring of visions of the 'sons of forgotten kings walking in loneliness, guarding from evil things folk that are heedless' (1.viii.146). He, too, had heard news and was waiting for them, though it was no plan of his to find them here (1.x.163-64).
When Frodo steps out into the larger world and takes on the task of saving the Shire, he leaves behind the place which defined him, where he was 'the Mr. Baggins of Bag End'; and he does so on the very night when it becomes clear -- to the reader if not immediately to Frodo the character -- that this identity is not quite the advantage it had long seemed to be, even within the Shire. Farmer Maggot's attitudes towards Hobbiton show this, as do those of most of the hobbits who discuss the 'queerness' of the Bagginses in the evening at The Ivy Bush and The Green Dragon. Mr. Baggins may find them 'too stupid and dull for words' at times, but behind their deference they have their own opinions of how strange he and Mr. Bilbo are. When Maggot links Frodo's present troubles to Bilbo's adventures, he is doing no more than voicing to Frodo's face the longstanding common opinion that no good could come of adventures to the 'queer' folk who went on them.
The larger world in which such adventures take place is far more dangerous in fact than even the most parochial hobbit imagines. Even the more broadminded Mr. Baggins of Bag End fails to grasp that not only is he 'quite a little fellow in the wide world after all', but that the wider world, whether it is the Faërie of The Old Forest, Bombadil, and the Barrow-wights, the world of History, or that blending of both in which a man might walk, will not be fenced out forever. The Ring, which threatens Frodo's identity because he already cannot do with it as he wishes, compels him to leave the place that helps define that identity.