Yes, you can file outland. Outland applications through the U.S. are (almost always!) faster than inland applications. With an inland application, you can apply for an OWP (open work permit), which will allow you to work after stage one approval. There is no OWP option with an outland application. However, this may not actually be of much benefit, as outland applications are often finished in the time it takes to get stage one approval on an inland application.
If you file inland, you are *supposed* to remain in Canada for the duration of the application. With a study permit, you're *probably* fine to go to the U.S. and return. If, for some reason, you were denied entry to Canada, an inland application would be considered abandoned and you would have to start over. Unlikely in your case, but only you can decide how "risk tolerant" you are. With an outland application, even if you were denied reentry, your application would continue to be processed.
You're right in that even an outland application is unlikely to be processed by April. If you can apply for PR in December, and then also apply for a PGWP, that definitely sounds like your best bet. I don't know anything about the PGWP, so you would need to make sure that you can apply for one with a PR application in progress (I would think you could, but I have no actual information about that).
If I were you, I'd opt for an outland application. I kicked myself for MANY months for filing inland. The one drawback to an outland application is that, if they require an interview because they doubt the genuineness of your relationship, you would be required to travel to your U.S. visa office (think they only have two, NY and Los Angeles) for that interview. You can likely avoid an interview by putting together a strong application with a solid proof of your relationship.
