If my PR card renewal is not approved and I have to enter Canada (can't wait for TRAVEL DOCS), can I enter Canada by driving a rental car from US? Shall I show them Confirmation of PR, expired PR card, passport and maybe printout of renewal status or any CIC mails?
This is completely wrong. You can certainly drive a rental car across the border. There are no issues with that as CBSA doesn't care how you get there. What do you mean by "it states very clearly"??
Once at the border, showing COPR and passport is good enough to be allowed entry as a PR.
Another option is to have a friend/relative of you to drive you to Canada side border: then once you in Canada soil you may rent a car or take the greyhound.
Stop making your own stuff up. A rental car works, I have done it many times at Windsor. In your case, I would do a one way rental. You could also choose to cross the border on foot at the Niagara.