If Swann’s Way zigzagged between Marcel’s childhood and Swann’s earlier affair, with clearly defined angles, Within a Budding Grove is corkscrew-shaped: each gnarled observation doubles back on itself and intersects with everything else, with no clear resolution.
One of the most vivid juxtapositions comes in Marcel’s view of Gilberte (the Swanns’ daughter), who has come to embody both the present and the past of her parents:

On Gilberte’s face, at the corner of a perfect reproduction of Odette’s nose, the skin was raised so as to preserve intact M. Swann’s two moles. It was a new variety of Mme Swann that was thus obtained, growing there by her side like a white lilac-tree besides a purple…It was when she had been to her classes, when she must go home for some lesson that Gilberte’s pupils executed that movement which, in the past, in Odette’s eyes, had been caused by the fear of disclosing that she had opened the door that day to one of her lovers, or was at that moment in a hurry to get to some assignation. Thus did one see the two natures of M. and Mme Swann ripple and flow and overlap one upon the other in the body of this Melusine. (607)

The main point here is how Gilberte takes after and intermixes parts of both of her parents, good and bad, but beneath that, there is also the suggestion of natures past and present: Swann’s abandoned social climbing, Odette’s coquetry, Mme Swann’s more subdued role as a wife, the resigned intellectual that Swann has become.
What of the two parents? Odette and Swann have ended up married, more seemingly by default than for any other reason. He doesn’t love her; whatever appeal she once held has not returned. Everything is for convenience. That is the immediate implication. But, as elsewhere, their present relationship affects the past as much as the past affects it:

The laborious process of causation which sooner or later will bring about every possible effect, including, consequently, those which one had believed to be least possible, naturally slow at times, is rendered slower still by our desire (which in seeking to accelerate only obstructs it), by our very existence, and comes to fruition only when we have ceased to desire–have ceased, possibly, to live. Was not Swann conscious of this from his own experience, and was there not already in his lifetime–as it were a prefiguration of what was to happen after his death–a posthumous happiness in this marriage with Odette whom he had passionately loved–even if she had not attracted him at first sight–whom he had married when he no longer loved her, when the person who, in Swann, had so longed to live and so despaired of living all his life with Odette, when that person was dead? (508)

Here, Proust presents Swann as a changed man: in fact, it was only through his change (his renouncing of his love for Odette) that he was ever able to marry him. Yet the marriage, despite his lack of love for her, makes a sanguine postmortem for Swann’s dead self, the one who did love Odette, in that he (the current Swann) has made his dead self happy by ending up with Swann, even if his current self is, at best, resigned. The irony arises in the fact that Proust presents it as a prerequisite that Swann had given up all his passion before he could have married her. The arrangement between M. Swann and Mme. Swann requires that he not be the crazed obsessive of years past but a reticent, somewhat cynical man of good standing. And so he is:

There was no renunciation on Swann’s part, when he married Odette, of his social ambitions, for from those ambitions Odette had long ago, in the spiritual sense of the word, detached him…In so far as a mental picture which accompanies one of our resolutions may be said to motivate it, so it might be said that if Swann married Odette it was in order to introduce her, together with Gilberte, without anyone else being present, without, if need be, anyone else ever coming to know if it, to the Duchesse de Guermantes. (506-508)

And also:

For a long time now it had been a matter of indifference to him whether Odette had been, or was being, unfaithful to him. And yet he had continued for some years to seek out old servants of hers, to such an extent had the painful curiosity persisted in him, to know whether on that day, so long ago, at six o’clock, Odette had been in bed with Forcheville. Then the curiosity itself had disappeared, without, however, his abandoning his investigations. He went on trying to discover what no longer interested him, because his old self, though it had shrivelled to extreme decrepitude, still acted mechanically, in accordance with preoccupations so utterly abandoned that Swann could not now succeed even in picturing to himself that anguish–so compelling once that he had been unable to imagine that he would ever be delivered from it. (564)

Again, there is the zombie Swann, who still acts on his old desires. And again, I’m not sure how much to trust this, since there is so much confusion and suggestion that these passages are only the temporary rationalization of Swann’s current self. It’s enough to say that Swann does not love Odette, certainly not like he did in the past, but he is still moved by her, and by his former self.
Most notably, Swann has a new, unnamed lover, who dredges up the old anxieties:

For between Swann and the woman whom he loved this anguish piled up an unyielding mass of previous suspicions, having their cause in Odette, or in some other perhaps who had preceded Odette, which allowed the ageing lover to know his mistress of to-day only through the old, collective spectre of the “woman who aroused his jealousy” in which he had arbitrarily embodied his new love. Often, however, Swann woulud accuse his jealousy of making him believe in imaginary infidelities; but then he would remember that he had given Odette the benefit of the same argument, and wrongly. And so everything that the young woman whom he loved did in the hours that he was not with her ceased to appear innocent. But whereas at that other time he had made a vow that if ever he ceased to love the woman who, though he did not then know it, was to be his future wife, he would show her an implacable indifference that would at last be sincere, in order to avenge his pride that had so long been humiliated, now that he could enforce those reprisals without risk to himself, he no longer wihsed to do so; with his love had vanished the desire to show that he no longer loved. (565)

This is the most vehement passage of renunciation, where Swann has not only given up on Odette totally, but he has also given up on even wanting to avenge himself on her by cheating on her. (He does it anyway, but he won’t tell her.) Yet Odette is still present in his fears about his new love, and it is not the “Mme. Swann” of the present day but the Odette of years past. Having been filled with emotion again, the memory of the object (i.e., Odette) that captivated him when he was in love is cast on to the new object (the unnamed woman) even as Odette herself is the subject of none of his old emotions. It is this remembered object, no longer extant, that informs his relationship with the new woman, more than anything in his marriage.
(I use the word “object” because he is so insistent on the myopia of each character as they interact with their projected, changing views of other people. It is to his immense credit that the subject changes so rapidly, particularly in “Mme Swann at Home,” as to produce vertigo and uncertainty.)
The themes are even a little trite here: Swann never got over old Odette, the green-eyed demon still torments him, he’s grown tired of Odette even though he got her. What’s striking is the treatment. It’s the tactic of laying out the contradictions over time, contradictions that don’t get resolved because there is no true consistency: Swann’s images of Odette in years past are, ultimately, his own, and they make themselves felt again because Odette then still exists for him, in that zombie part of him that can still react without reason.
Compare it to the end (ch. 17) of Joyce’s Ulysses, where there is the realization that Bloom’s relationship with Stephen Dedalus will not recreate Bloom’s dead son Rudy, due to “the irreparability of the past [and] the imprevidibility of the future.” Joyce’s faux “scientific objective” tone in that chapter serves to pull Bloom from inside his own head to a point of (as I always read it) despair, but also to a point where his endeavors may be cast in a nobler light. Proust focuses exclusively on the former aspect, but he never admits a single emotion above others. He is persistent in destabilizing his frameworks, and there is a certain humility in that. I find Ulysses heartbreaking, but I’m also inclined to think that emotionally, it is more tradition-bound than Proust.
Later, when Marcel is mourning the death of his love for Gilberte, he might as well be talking about Swann:

The picture of the beloved in our minds which we believe to be old, original, authentic, has in reality been refashioned by us many times over. The cruel memory, on the other hand, is not contemporaneous with the restored picture, it is of another age, it is one of the rare witnesses to a monstrous past. But inasmuch as this past continues to exist, save in ourselves who have been pleased to substitute for it a miraculous golden age, a paradise in which all mankind shall be reconciled, those memories, those letters carry us back to reality, and cannot but make us feel, by the sudden pang they give us, what a long way we have been borne from that reality by the baseless hopes engendered by our daily expectation.
But after a time, absence may prove efficacious. The desire, the appetite for seeing us again may after all be reborn in the heart which at present contemns us. Only, we must allow time. But our demands as far as time is concerned are no less exorbitant than those which the heart requires in order to change. For one thing, time is the very thing that we are least willing to allow, for our suffering is acute and we are anxious to see it brought to an end. And then, too, the time which the other heart will need in order to change, our own heart will have spent in changing itself also, so that when the goal which we had set ourselves becomes attainable it will have ceased to be our goal. Besides, the very idea that it will be attainable, that there is no happiness that, when it has ceased to be a happiness for us, we cannot ultimately attain, contains an element, but only an element, of truth. It falls to us when we have grown indifferent to it. But the very fact of our indifference will have made us less exacting, and enables us in retrospect to feel convinced that it would have delighted us had it come at a time when perhaps it would have seemed to us miserably inadequate. One is not very particular, nor a very good judge, about things which no longer matter to one…So that we can never be certain that the happiness which comes to us too late, when we can no longer enjoy it, when we are no longer in love, is altogether the same as that same happiness the lack of which made us at one time so unhappy. There is only one person who could decide this–our then self; it is no longer with us, and were it to reappear, no doubt our happiness–identical or not–would vanish. (675-676)

In the second paragraph, the first third or so are things we’ve heard before. It’s the looping back (from “Besides…” onward), the delving into a hypothetical past space where an individual subjective mind cannot exist, that is novel.