It's part of the New Aesthetic. Take something that should be high res and make it low res on purpose and voila, a masterpiece that is perpetually new and old at the same time.
They do! Horns are solid bones! Unless they are filled with somthing other than bones, then they are not solid bones. However, as long as they do not have another substance inside of them they are indeed solid bones.
You have taken time to count the dragon bones, that is dedication good for you I would not even attempt it I have zero patience and I would not be able to do that.
I doubt bethesda made the dragon skeleton bone by bone, they would have made as many bones as required to make the dragon model work. I'm sure lore-wise they are meant to have more bones