What is it like to be a bat?

Dennett has a character telling a story about a bat:

Here's Billy the bat perceiving, in his special sonar sort of way, that the flying thing swooping down toward him was not his cousin Bob, but a eagle, with pinfeathers spread and talons poised for the kill!

He then points out that this story is amenable to criticism. We know that the sonar has limited range, so Billy is not at least perceiving this eagle until the last minute; we could set up experiments to find out whether bats track their kin or not; the sonar has a resolution and if we find out the resolution we know whether Billy might be perceiving the pinfeathers. He also mentions that bats have a filter, a muscle, that excludes their own squeaks when they pick up sonar echoes, so we know they aren't hearing their own squeaks directly. So, we can establish lots about what it could be like to be a bat, if it's like anything. Or at least what is isn't like.

What is that criticism supposed to be criticizing?

Nagel's paper covers a lot of ground, but none of what you described has any bearing on the point about it "what it's like" as a way to identify conscious experience as distinct from, say, the life of a rock. (Assuming one isn't a panpsychist who believe that rocks possess consciousness.)

The pessimism, the "facts beyond the reach of human concepts".

It gives obvious examples of the way our awareness of factual circumstances give us inroads into what might be experienced. And the upshot is that this might suggest the rest of consciousness can be understood by iterating forward in a similar manner. Dennett makes this exact same point about Mary's Room. Far from talking past the article, it's attacking the fundamental principle.

That's the magic answer. It's a/the hard problem, but permeable to inquiry. The top neuroscience research into consciousness however doesn't seem like this kind of inquiry Dennett is referencing.

"Might suggest" - sure, that might be possible. That's not so much a criticism of Nagel as the hope that the problems he's highlighting might one day be solved.

That's not pessimism. It's interrogating the nature of reality. If you want to discover truth, you can't shy away from conclusions because they don't make you feel good.