This plugin displays debt reduction calculator
Yes, you can change it in Plugins/Debt Reduction Calculator page
Yes, you can put Adsense or any other ads
It's not required but a link to http://calendarscripts.info/debt-reduction-calculator.html will be highly appreciated.
Because they weren't available when I first wrote the plugin, and I have to keep the format for backward compatibility. Technically it makes no difference, really.