What are my chances of being able to come back in Dec?
Only the IO that you meet at the border can answer that question with 100% certainty.
The IO you met before told you that you had to establish yourself in the states, he/she is likely to have placed a note on your electronic file.
Read the notes for visitors at the CIC Web site, it will help you prepare for your visit.
Why do you want to wait to apply for PR, do you intend to file common law?