questions about chapterbooks and MG/xmas stories

E.T.A. Hoffman wrote that in 1816. It's a classic, but it has nothing to do with the current MG and chapter book market.

As far as your other questions, I'm pretty sure there's info in the sticky threads about word count and target audience. Chapter books are for kiddos who are just starting to read longer books--usually late first grade through second grade, beginning of third. Middle grade is longer and more complex in vocabulary and sentence structure. It covers a wide range of readers from 8 - 12; sometimes up to 14 for upper middle grade.

In addition to what Ruth said, if you're interested in writing for children, go to the library and check out several newer releases (last five years or so). Read, read, read. That's the best way to see what works - and what doesn't - in the current market. And it's fun.

My hunch is that it's near impossible for lower MG, too, if it's truly about Christmas.

The trouble is, people usually only buy Christmas books around Christmas-time, and not too many of them. At most, a kid will probably get a couple of books at Christmas, and they won't want them all to be 'Christmassy'.
There's a lot of holiday classics out there that people remember from their own childhoods, and will buy first.
And, if a child has been getting Christmas picture books, elves, snowmen, reindeer and Santa have been pretty well covered. (Also, little lost 'anything' that finds it's way home or a new home, at Christmas.)
Yes, a chapter book is different from a picture-book, but is it going to be different enough?

Chapter books usually come in series. Stand alones are hard sells without the holiday theme. There are holiday themed book in many chapter book series. You're competing with the known characters. Take a look at chapter books in your library or search Amazon for Christmas books for the age group. Everything that comes up is your competition.

I have a chapter book about a Tooth Fairy. I hear the question, "do your readers still believe?" That question will com up for you too. My answer is, those that don't often still wish they did and enjoy the stories. They also enjoy the cash.

Elf on a Shelf began as a self published book because holiday books are tough sells. It has become a new classic. Read its story and see if you feel you have the mojo to create that kind of magic.