How about cat food? We always keep open cat food tins in the fridge with a lid on otherwise maggots develop if we leave it out on the side. Am I slowly killing my cat?
I suppose it's ok. Animals have a higher resistance to bacteria than we humans do. For instance, they have no problem eating raw (sometimes decaying) meat.
Yes, but not if the can is dented. The inside of cans are coated to protect the contents from the metal and possible reactions to said metal. If the can is dented, the protection is broken and reactions can occur that are less than optimal.