1. Can I complete the remaining process at Pearson Airport while coming back from USA once I receive my passport from Ottawa with PR visa & CoPR? or do i really have to go to Rainbow bridge at Nigara fall for completing the remaining process or can it be done at YYZ airport as well?
Yes, You can do the landing process at the Pearson airport while coming back from another country [You can do it at any port of entry]
2. After receiving the passport with PR visa from CIC, what exactly actions are performed at flap poling at port of entry?
If you chose to do the landing at a land border then you can do flag pole. Just go to the US side of the border and tell the officers that you are there for landing as a PR in Canada. Then they will give you a document and guide you to the Canadian border. You have to give your COPR, passport and the document got from the US at the Canadian border and the officer would give you an yellow paper along with other documents and ask you to park your car and go the immigration office. [In my case I had US visa so I didn't flagpole instead went to see the Niagra falls at the US side and landed on the way back, in this case no document is issued by the US immigration]. You can also cross the Rainbow bridge by walk if you prefer that over driving.
3. When we can apply for PR card and what is the process of that.
You don't need to apply for the PR card, when you are landing it would be applied automatically and the officer will verify the address with you.
4. Since I already have valid TRV with multiple entry, so will CIC still stamp PR visa on my passport or they will issue just issue CoPR letter?
Even though you have a multiple entry TRV, CIC will stamp a single entry immigrant visa on your passport. When you are landing the immigration officer will cancel that visa. Your TRV and work permit become invalid [though they don't cross it or shred it]. Once you landed it would take ~57 days for you to get the new PR card. If you travel outside Canada after landing but before getting your PR card : "Permanent residents (PR) of Canada must carry and present their valid PR card or permanent resident travel document (PRTD) when boarding a flight to Canada, or travelling to Canada on any other commercial carrier. If you do not carry your PR card or PRTD, you may not be able to board your flight, train, bus or boat to Canada." But you can drive from US in private vehicles.