Errors that have been corrected are bolded:
"Cosmo Fairywinkle-Cosma stopped the actress' dog from running into the road."
When using possessive apostrophes, if the word ends in an 's', all that is needed is an apostrophe at the end of the word. It is improper to have an extra 's' after the apostrophe.
Example:
Incorrect: The dogs's food was freshly served.
Correct: The dogs' food was freshly served.
Hope this helps a bit. Best of luck to you, mate. :)