The Benwick-Louisa romance is indeed a handy plot device, but I think it is perfectly credible. Let's consider the protagonists:

Louisa is a lively but ordinary young woman who is looking for a husband. As I've posted before, I think she is fairly immature and unformed, ready to adapt herself to whichever partner she winds up with. Wentworth is attractive and available, but he does not single her out particularly from her sister; rather it is Louisa who makes the running, as we see on the walk to Winthrop:

"Louisa certainly put more forward for his notice than her sister." (10)

Her efforts appear to be working, and she is open to suggestion. After Wentworth's hazelnut speech Louisa is "armed with the idea of merit in maintaining her own way," (11), which leads to her fall on the Cobb. She cannot just be herself with him , but tries to shape herself into the sort of woman she believes Wentworth wants - theirs was not a meeting of souls.

So I don't think Louisa was deeply in love with Wentworth, and I'm not surprised that her affections turned so easily to Benwick. Wentworth may have seemed unnervingly glamorous to Louisa - perhaps a little out of her league. He offered her philosophical speeches about hazelnuts and seemed to want her to be something she was not. Benwick fell for her when she was ill and not looking or feeling her best; he offered her poetry and passion, and he wanted to marry her. No contest!

When we first meet Benwick he is Grieving with a capital G for the death of his fiancee:

"Captain Wentworth believed it impossible for man to be more attached to woman than poor Benwick had been to Fanny Harville, or to be more deeply afflicted under the dreadful change. He considered his disposition as of the sort which must suffer heavily, uniting very strong feelings with quiet, serious, and retiring manners."

Benwick is evidently a man who shows his feelings, but Wentworth is evaluating him from the perspective of someone who has kept his feelings hidden and after eight years is still not reconciled to the loss of the woman he loves. Perhaps Wentworth is projecting some of his own personality onto Benwick here.

I'm sure Bewick's affection and grief were genuine but I wonder if he is to some extent 'acting out' his feelings, both because this is his style, and because it suit his self-image as a man of deep romantic sensibility. I also wonder whether he has been unconsciously reluctant to abandon the more florid expressions of his grief because he has been living with Captain Harville, who's sister it is they are both mourning.

Everyone treats Benwick as though he is fragile because of his present misery, but fragility is not necessarily his natural state despite his penchant for poetry. Admiral Croft says of him:

"An excellent, good-hearted fellow, I assure you; a very active, zealous officer, too, which is more than you would think for, perhaps, for that soft sort of manner does not do him justice." (18)

As soon as she meets Benwick, Anne intuits that he will not be forever inconsolable.
"And yet," said Anne to herself, as they now moved forward to meet the party, "he has not, perhaps, a more sorrowing heart than I have. I cannot believe his prospects so blighted for ever. He is younger than I am; younger in feeling, if not in fact; younger as a man. He will rally again, and be happy with another."

Maybe Benwick's 'romantic radar' is beginning to function again and she picks up on that, as she surely does when he seeks her company several times at Lyme.

But what about the timing? Is it not too soon for Benwick to be truly ready for a new relationship? He received the news Fanny Harville's death at the beginning of August; it is now late November, so he has been mourning for nearly four months.
Grieving is a natural process that typically proceeds by stages. It takes time, but it's duration is highly individual. The acute phase may be over in four to eight weeks, and the whole process can last a few moths or last a lifetime.

Grieving is harder if a person is isolated and has no support - Benwick has had the close support and continual company of his friends from the moment he heard of Fanny Harville's death. Grieving is also harder for people who have difficulty expressing their feelings, which certainly does not apply to Benwick.

Benwick had been engaged to Fanny Harville for a year or two, for most of which he must have been at sea, so although he was deeply attached to her his life had not been shaped by he as it would have been had they been married and in each other's company every day and night. The duration of his mourning is within normal parameters, and may have been shortened by the ease with which he expresses his emotions and by the support of his friends. I see no reason why he should not be ready to love again.

Anne "..... was persuaded that any tolerably pleasing young woman who had listened and seemed to feel for him would have received the same compliment. He had an affectionate heart. He must love somebody." (18)