A couple of points in the question need clarification. First, Elinor's thoughts are not about Marianne's reaction to Willoughby's letter. They are in response to Marianne's view of Mrs Jennings, their well-intentioned but interfering hostess in London. Marianne is convinced that Mrs Jennings' kindly interest in her situation is simply driven by a desire for gossip. Elinor thinks Marianne shows injustice toward Mrs Jennings.
Second, Marianne does not have a "deficiency of ... sensibility." On the contrary, she has far too much of it. Here is the definition of sensibility from Merriam-Webster:
1 : ability to receive sensations : SENSITIVENESS
// tactile sensibility
2 : peculiar susceptibility to a pleasurable or painful impression (as from praise or a slight) —often used in plural
3 : awareness of and responsiveness toward something (such as emotion in another)
4 : refined or excessive sensitiveness in emotion and taste with especial responsiveness to the pathetic
Elinor thinks of Marianne as oversensitive, and says Marianne expects others to be just as sensitive as she. This leads her to be unjust in her judgment of others such as Mrs Jennings who are not as sensitive in their feelings or as refined in their manners.
Elinor says that in this, Marianne is like half the rest of the world. The quoted passage goes on:
She expected from other people the same opinions and feelings as her own, and she judged of their motives by the immediate effect of their actions on herself.
In other words, Marianne expects everybody to see things the same way as she does, and condemns those (like Mrs Jennings) whose feelings differ from her own. Marianne also cannot see beyond herself, and judges people's actions not on intrinsic merit but on how those actions make her feel. This makes Marianne like half the world, which also misjudges people similarly. According to Elinor, this is neither reasonable nor candid. Candid here is used in sense 2 in Merriam-Webster:
free from bias, prejudice, or malice : FAIR
// a candid observer
Understanding the passage in this way, we can deduce what if more than half there be that are clever and good means. Elinor thinks that half the world is like Marianne in being clever and good, yet unjust in her judgments of other people. But there are also clever and good people who do not judge people unfairly; for example Elinor herself. So if half the world is like Marianne, and additionally there are some people like Elinor, then more than half the world is "clever and good." We cannot just assume this, so the qualification comes in: if more than half are clever and good, then Marianne is like half the world.
This is an example of Austen's sharp yet subtle irony. Without flat-out saying that most people are stupid and/or evil, she implies it by saying, "Marianne is like half the world in that despite being clever and good, she still misjudges people. Well, assuming that more than half the world is clever and good."