I think a new checkpoint is not a bad idea in principle. But you don't want to write too many blank checks to other working groups. I think the change is ok. We would need to define it. In definition of caption, make clear that in context, may refer to table captions not accessibility captions.
Braille is an abstraction.
There are implementations with contractions. Remove from defn of natural language. But refer to Al's comments. This refers to alternatives, you would want the content synchronized. Simplify checkpoint 2.5.