You still have time to sort it all out.
Dropping off a rental car should not be a problem, might just add an extra drop-off charge.

Not directly answering all your questions. I would prefer to drive to Niagara Falls if I already have a rental car. There are plenty of metered parking spots near the Falls. In fact, you might want to stay overnight right there.

In Toronto, among other things you want to do and see, not to miss a ferry ride to the islands on Lake Ontario.

Here is a link to a travel journal on a very similar trip as yours. Might be helpful:

https://stefanofoto.smugmug.com/Trav...nada-Road-Trip

Cheers