What geothermal savings calculator are you using? The ones online are way to basic they just assume a percentage savings. To be accurate you need load calculation numbers(both heating and cooling) the efficiency of the new system, temperature data(for the savings calculator) for your specific area, your electric rates and gas/oil rates.

A properly installed geothermal system should not need backup heat unless you were to loose power or go into an extreme low temperature situation(we usually install electric because our systems will never go into backup unless there is a breakdown or broken loop). For your water heating they should be using a desuperheater and probably a storage tank for your system. Where are you located? How much per year do you spend on oil currently and what is oil per gallon$$$