I think electronegativity, molecule structure and a whole bunch of things add up to the energy before and after. Just taking the orbital in account is not enough to conclude that it will bond or not bond. The reaction will happen if the gibbs free energy drops. Your argument is that for...
Latent heat of anything can simply be calculated by taking the difference between enthalpy after and before.
I dont' remember but the dependence of the enthalpy before and after will tell you if it will change the latent heat.