4. Live in GTA, ON, mailed application to Gatineau, QC, passport issued from Kelowna, BC (wild!)
5a. Apply by mail if you're in no rush to get your passport.
b. Passport and Service Canada offices are open on non-statutory holidays. Most people don't know this so it's not as crowded on those days.
6. No. I have also been a guarantor 4X and I have never been called.
Does paying more also mean buying a flight ticket or taking a hotel reservation in US? The fastest one only applies in case of emergency, it's not available if you "just" want to get your passport asap.
Also, both urgent and express application are only available in person...
The decision should be quite easy. When are you leaving Canada?
If it's not anytime soon, getting the passport earlier is kinda useless to be honest.