Well, the old coolant is on the inside of the engine. Externally in the VOD, the problem is when you have dead leaves and other dead vegetation that collects. When it rots, it will lower it's pH and start to corrode the aluminum. That's the problem. It also gets made even worse if you're leaking coolant from the Y-Pipe/water pump when the coolant collects there.

As for the alternator, don't sweat it. I've driven through flood water where I've submerged the entire thing. The warning light will come on, but I just gun the engine until thing just flings out all the water and it's good to go again. Even been through an industrial waste spill and even raw sewage with no problem.