a) the higher the enthalpy of formation of a substance, the higher the energy level of the substance.
The higher the energy level of the substance, the less stable it becomes, as it can lose more energy by reacting to from lower energy products.
This, however, only refers to thermodynamic stability, not kinetic stability, which is unaffected by enthalpy of formation.
b)I'n not sure on the difference, but the more thermodyamically stable allotrope is the one which is at a higher energy level, and will therefore have a larger enthalpy of combustion.
Hope that helps.