If there is R space when you book your B ticket (if you're doing this 2+ weeks out, there usually will be), then yes, you can book and insta-upgrade; and yes, it's often cheaper than P fares.

Yes, you only get 125% AQM instead of 150%.

If everything goes smoothly you'll be fine, but a B ticket is ultimately a B ticket, so if Air Canada decides to change planes or you need to be rebooked (a plane goes mechanical, weather delays make you miss a connection, etc.) you might end up back in the Economy cabin.

You can't upgrade using eUps which expire before the flight in question, so if you're looking at flights in March 2017 you won't be able to upgrade them yet.