There are chemical cleaners that can take care of that, but it doesn't seem to be particular bad in your case. For light corrosion like that, rubbing them down with a pencil eraser is usually enough to clean up the contacts and restore the circuit.
Yeah as said above but light sand paper helps if it is bad. You just want batteries to make contact so sanding helps a little but do it lightly and slowly.
Yeah that’s not bad. All alkaline batteries will eventually leak so it’s best to take them out of something if you won’t be using for a while.