I am not a professional at all so please confirm all of this, but I have been in your situation.
1) I was able to cross at Niagara in a rental car with my ORIGINAL Landing Papers. I also had my (recently) expired PR card - and I printed out the notification that Immigration Canada had received my application and it was being processed. The particular crossing official I got was pleasant and barely looked at my papers. (To be safe I had brought a copy of my application with all supporting docs.) But I assume other's mileage may vary. (And, times being what they are, it's probably worth noting I'm a YT Am-born Male.)
2) What I was told is that you should avoid entering Canada as a visitor if you are intending to retain your PR status. I would guess it is exceptionally rare, but I was told that Canada Immigration has used custom docs that indicate a returning resident is a "visitor" to claim they gave up PR status. Again, this is 2nd hand and I have no idea if it's true or even common. It's probably best, if you can avoid it, not to "pretend" to be a tourist to get on a plane, train, bus, boat - but I also bet many American Canadian residents do it and few have any real issues. I'm just a total 'by the book' guy.
3) You can get returning resident permits if you are out of Canada when your card expires. It is a good idea to bring digital or physical copies of your application with you as they ask for some of the same info (or, better yet, download the returning resident application before you go, and make sure you have the docs they ask for.)
Hope this helps. If you are going to Toronto anyway, flying to Buffalo used to be the easiest thing, but the one hiccup now is rental car agencies have stopped the Pearson drop-off for rental cars. That may change with the opening of the border, but at the moment it seems the only option would be: getting some kind of transpo to a crossing you can walk across, then get a cab or other to a Canada based rental car - or, have someone come from Canada and pick you up.
Again - I'm not a pro - just a long-time Can res who has been through the PR Card dance few times (and am about to get skunked again due to the long turnaround times.) as always YMMV.