1) That's right, you do not need to provide a Canadian PCC, regardless of how long you've lived here, because IRCC is able to perform a Canadian-relevant background check internally. Source:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=037&top=4
2) The issue is, IRCC provides updated information in the request letter they send you which contradicts the information you wrote here. In the request letter they say "Where you have spent most of your adult life" I believe they say this now as a vague statement so that you do not have to provide PCCs for countries where you've been going on and off for short vacations and haven't actually lived there properly. They say that if there's a country they need it from, they will ask you specifically. i.e You don't have to follow the 6 month rule as of right now. BUT, from a practical standpoint, it depends on the procedure and time I guess. If that country's PCC is extremely easy to obtain quickly, I would just add it in case. If it'll take a while, then you can go without it and see if they ask for it later.