Why exactly does molten NaCl explode, when it is poured into water?
Understand that sodium chloride is not made up of sodium metal and chlorine gas but of sodium ions and chloride ions, held together by ionic interactions. Under these thermal conditions (liquefication), the compound will not decompose into its elements and therefore all reactions you suggested which include elemental chlorine or sodium cannot occur.

Dissolution of sodium chloride in water is neither strongly exothermic nor strongly endothermic, so any effects stemming from the dissolution are neglegible.

Also note that you neglected an important variable in your calculations. As far as I can tell, you only calculate how to arrive at melting-point hot solid sodium chloride. To liquefy, additional melting enthalpy has to be applied, a further reservoir of energy to draw from. This melting enthalpy is of course released upon rapid crystallisation, but you should really subsume it into the heat energy difference altogether.

Finally, thermal decomposition of water is not exactly a process that will happen quickly, as exemplified by (water) steam temperatures that can be huge. Check out this unrelated answer of mine about how much energy is required to heat water to $100~\mathrm{^\circ C}$ and boil it.

So all things considered: The principle contribution to the explosion is the rapid heating of water including boiling, leading to tightly compressed gaseous $\ce{H2O}$ which expands explosively. This concurs with the recrystallisation of $\ce{NaCl}$ which accounts for a non-neglegible portion of the energy added, but which is technically already present in molten $\ce{NaCl}$. All other processes are minor or impossible.