Yes you can apply for another extension. you will have to met all requirements to re-enter, once you leave Canada. There is no visa for visa exempt countries. The thing is you will be common law in October so, if by December you have all the information filled out and documents together pay the fee and send in your app (file out-land so you can freely cross, inland you must stay see page below) that will give you a PR fee receipt and send your app by courier you will have receipt that app has been mailed. This way when crossing back over your showing them that you intend to do everything legally. They will see you crossed before an applied for extension and have now filed for PR.http://www.cic.gc.ca/english/information/applications/guides/5289ETOC.asp
Leaving Canada can automatically cancel temporary resident status as a visitor, student or worker.
edited to say: this won't guarantee your entry, but it will show your telling the truth.