Hi ecoptimist,
Unfortunately, you will only get 2/3rds of it. The $17,500 is for a 12 month period.
As for the discussion on when to take the SSHRC award--either in May or September, I'll be entering my second year of my MA this Fall, with my SSHRC starting this September. After seeing this post about taking a May over a September start date, I panicked, so I went to both my awards office and department about this matter (according the SSHRC's website, you can change the date). From what I was told by awards and my department, many MAs do have to defend their thesis over the summer, so it's wise to have your funding still in place over that time. If one is accepted in a PhD program for the Fall (which people normally are well before they even finish the program, usually in March), the universities are already aware of the summer defence period and do wait for you on conditional acceptance. Hopefully this advice I was given was true, and if anyone else knows otherwise, please let me know!
P.S. I agree with Ludwig though--the universities do make this very unclear and you really have to ask. Thanks for starting this discussion!
Cheers!