you can read the first page of this thread for the advantages of inland and outland, and see what is best for you.
You can apply outland even if you are in Canada.
If you apply inland, you will have implied status as soon as you send your application, so no need to worry about extending your visa. BUT - you can not leave Canada for any reason during the whole time ( currently 6 months first stage + 8 months second stage) otherwise your application will be deemed abandoned if you're refused entry at the border.
If you apply outland, you will have to extend your visitor visa ( online extension, do it before the current visa expires, and include proofs that you have applied for PR).
About getting into the country - it's always up to the border officer. I got in with no return ticket, and a PR application already started, and have explained that I was coming to wait out my PR, and it wasn't a problem. I had proofs of funds ( because no return ticket), and proofs of PR application started. But - there is no guarantee... it's best to be honest about your intention at the border. If you're married and decide to file outland, you can do it as soon as you have your marriage certificate, and have the proofs that you have applied when you enter Canada (the fee receipt, or the post tracking number to send your package to Mississauga).
Good luck with the decision,
Sweden