Hi raft,
Applying for extension is easy process, online vs paper based (ur choice). As per CIC website, online is taking around 28 days and paper based takes 110 days.
My spouse is here in canada on tourist visa (Uponfirst entry stay validity granted was for 6 months). We filed for extension in May beginning (paper based).
The reason for paper based is if in case her extension is refused we got time (approx 110 days) to stay together. This is what we wanted to opt for.
Yes while filing extension, you have to prove that you will leave the country on time and will remain in country legally etc...
I am still waiting for decision which might come in august end or september. Hope am able to provide you some clarification.
Thanks,
Jul2014