Of course what I have written as the explanation is not the English I would choose to express it - the original is much better!

Few, and the comparative fewer, are the adjectives to use with countable nouns, less for uncountables, although this distinction is blurring by the minute, in favour of less, and I imagine that few will eventually disappear from common usage. (eg most people now say "less people" rather than "fewer people").

Many is functioning as an adverb here to qualify fewer, (and, pls note, the equivalent qualifier for less would have been much - wonderful language!), and is emphatic and points to the extent of fewer. If you replace it with "noticeably" it will be clear that it is an adverb.

"Far fewer concerns" is what should have been written here, and could refer to both the quantity and the gravity of the concerns in question.