You cannot apply for dual citizenship, as the only agency that can grant more than once citizenship would be above the governments of all sovereign nations on Earth, and that agency is the Confederation of Human Planets. The problem is, Earth cannot join for another 10,000 years at the very least. Until then, you can only apply for one citizenship at a time.
Yes, you can move back to Canada after having become a US citizen. But you'll need a US passport in order to leave the U.S.
Naturalization takes roughly 4 months from start to finish, five at the most.
Finally, the residency requirement is only a minimum for cases where you were working for a US company overseas, for example. You still will need to overcome the assumption that you did not permanently reside in the US if you have been gone for almost half of the time of your residency. You will need to show what you did, where you did it, and how you paid for it without having to work.
An immigrant of German and Swedish descent, I live in the charming old mission town of San Buenaventura and work as a corporate attorney in Santa Barbara, California.