I'm not very good at grammar, but I thought past participle clauses were like:
Founded 50 years ago, the company has had half a century of success.
Pleased with her essay, she submitted it to the teacher with confidence.
I don't see such a formation in the examples you gave. In the original example, I wouldn't include a comma:
Communication with the outside world was knocked out leaving overseas relatives frantically trying to contact their loved ones.
If I'm not mistaken, 'leaving' here isn't a verb, it's a noun (gerund). Is that right?
You could include 'so' as you did, but it would change the structure:
Communication with the outside world was knocked out, so relatives were left frantically trying to contact their loved ones.
You'll need someone who has better knowledge of grammar to explain why this is so...