MIT should be good for this project, also Creative Commons can cover your needs. Github should be good to host it, so you can manage your repo with issues and pull requests.
Nice, I'd use it. Having the ability to search by address would save a lot of zooming though. Is this just a straight overlay on the existing Google Map widget (which would make customizing easy)?
Be careful with drawing multiple polygons -- each one stays on the map, but the kml export only includes the last one you drew.