If iam a canadian graduate... can I chose that the degree has been awarded and support this with a letter of completion showing that I will be granted my certificate later in the convocation ceremony??!
Hmm. 415 is not that high. If it was 430+, I'd recommend rejecting it, but with a 415, it's a tougher decision.
In your place, I would spend a couple hundred and consult an immigration attorney. The forum is really helpful, but in edge cases like yours you cannot be completely certain that you're getting the right advice. I always do my own research first, then I ask on the forum. It helps that I actually have a law degree and therefore I'm comfortable and confident reading legal provisions and making an interpretation. However, I'm not a professional immigration specialist, so I can't tell you what to do here.
If you are truly stuck (and you can afford it) go talk to a lawyer. For a decision this important, it's worth it.
Yes. All your school documents (ie Letter of Completion, transcript, diploma) should all show one date of completion. At least that's what I got when I completed my program here in Canada. As long as you have your letter with the daye of completion, you can claim the points for Canadian degree.
You might want though to wait until you receive your actual diploma (I got mine a few months before the graduation ceremony) so you can include it in the docs you submit in your application.