The tickets are not changeable so they will go ahead with the plan on oct 7th. If it doesnt work out then mom n dad will land first and will book another ticket for brother. Not sure for the other one we should wait till the pr card arrives in mail for dad?
So, you mean the plan is:
Brother will try to board in India - he may be allowed or not (you don't know yet)
Say if he manages to board (somehow), you are not sure if he will be allowed to clear immigration by the immigration official (what happens then?, return flight from Canada? same day? AC042?
I don't think its a function of waiting for the PR card. If your parents land, they become PR immediately when they sign the COPR in presence of the official and not when the PR card is issued. Your brother can follow immediately (moments) thereafter.
Your case is very interesting for sure; and somehow I don't think the CBSE official that you spoke to gave you the correct information.