When the job market improves and NATS realises that they are short of controllers (again!), then there would be no problem applying from Canada (on my course 4 years ago we had an Aussie, a South African and a Finn). I recall that you have to pay for your flights but NATS will pay for one night in a hotel prior to your interview etc.
On your second point; NATS are fairly flexible with your start dates. They will allocate you to a course but if for personal reasons you can't manage that date then there is usually no problem with you asking to 'slip' onto a subsequent course.
I'm not NATS HR by the way! This is just from my own experiences a few years ago. Give HR a ring (check out the website for contacts.)
NavCanada still not the best place to work then?!
Regards, ADIS