Although the word so 'this' meets the criteria, I think, of being considered a demonstrative, it does not contrast syntactically with the word ka 'there'. This has already been noted. Also, as some of the examples show, the function of so is not as strong as a demonstrative. Speakers are not using it to mean 'this' to distinguish a noun from some other noun in the discourse or context. It may have a textual function: to keep the audiences' attention focused on the participants of the story. It may also be a stylistic feature: another example of the great amount of repetition (or redundance) in the use of certain words.