Generally to clean up corosial I use a bristle brush or qtips dipped in the highest concentration isopropyl alcohol I can find. Once done with that I would leave the camera to sit for a few hours to make sure any remaining liquid dries up and evaporates.

I live in Arizona so this might be different depending on where you live. But for me batteries last about 2-3 years when I am using them. Depending on what type of battery you have I would say it is safe to say that if it has been sitting discharged for 2 years and it won’t hold a charge after being jumped. Then it’s dead and it’s time to get a new one. Check your local stores I know some places offer trade in credits towards a new battery. Hope this helps and sorry about the battery.