For the potential energy, you can set where zero is. It's the same way as if you set zero gravitational energy to be you standing on the earth, if you jump into a hole you have "negative potential energy".
For the second part, energy is conserved. E^2 = (pc)^2 + (mc^2)^2 where p is the momentum. If the decay products have momentum, they won't have the combined mass of the parent.