You are missing a small but important distinction between the things these two words are referring to.
Less than one-quarter, or 23%, of more than 2,000 adults polled earlier this fall said they felt “very comfortable” about their finances.
In this sentence, "Less than one-quarter" is referring to a numerical fraction, a statistic derived from the adults being talked about.
Fewer — just 12% — consider themselves wealthy, the report said.
In this sentence, "Fewer" is referring directly to the adults in question. The percentage given is in an incidental clause separate from the main grammatical structure of the sentence.
In the first sentence, "less" is valid for talking about the fraction or percentage, but "fewer" would also be valid as referring to the countable quantity of adults that the fraction or percentage is calculated from.
In the second sentence, considering the grammar without the dash-separated incidental clause, I think "less" might be technically valid, but would be awkward due to the length of the implied reference target. Inserting the reference to make it no longer implied would produce a sentence like this:
Less than 23% of adults consider themselves wealthy, the report said.
"Fewer" is more clearly valid and feels more natural to me. Only a single word would have to be inserted to remove the implied reference while maintaining grammatical validity:
Fewer adults consider themselves wealthy, the report said.
In the second paragraph quoted in the question, both references are in context similar to the first sentence. As noted for that, "less" is valid as a reference to the numerical fraction, and "fewer" is also valid as a reference to the countable quantity the fraction is calculated from. Which word to use in each place is just a stylistic choice.