It's an interesting theory, but I'm hesitant to give much weight to weakly-supported hypotheses intended to explain very broad and inclusive phenomena, like "murders (or a lack thereof) occurring within these arbitrary geographical borders." This is especially true when there's no shortage of plausible theories and a lot of potentially-useful information is missing.
The changeling myths seem to serve the purpose of guilt-relief only insofar as they also aid shame-relief, so I''m not sure they're all that helpful. (Am I missing something?)
Basically, this reads to me like an interesting but not particularly credible just-so story.
Related: The Psychological Diversity of Mankind, An African Folktale, many of the more interesting infanticide & abortion debates on this site
A fascinating post that however might need some background reading, most relevant material is linked in the article itself. I encourage reading up on the material.
Link to article.
Stories about changelings replacing babies and the recommended course of action being basically to expose the child is not a human universal, they are found only in European cultures. These rely more heavily on guilt and less on shame to regulate behavior than most other human societies. This may not be a coincidence. The stories look like they work as a ready made rationalization to reduce guilt from infanticide. Common problems often acquire common solutions like this.