As for the first and second questions, IANAChemist, and I don’t know the bonding energies of those things offhand. Whichever has more energy in the bonds masses more, since the matter is the same.
As for the third: Of course the mass doesn’t change. Nothing’s entered or exited the system as a result of eating the meals. If anything it gets slightly lighter because of radiating energy into the colder surrounding air, and burning off jet fuel, but those have nothing to do with the meal.
To within the precision to which they can be measured with any experimental instrument, 2 moles of H[sub]2[/sub] plus one mole of O[sub]2[/sub] will weigh exactly the same as 2 moles of H[sub]2[/sub]O. Assuming equal temperatures in both cases, the H[sub]2[/sub]O will in fact weigh slightly less, but the difference would be undetectably small. In your example of putting the gasses in a sealed chamber and sparking them, there would not even be that small difference of mass, provided you’re asking about the moment immediately after the combustion before any heat had a chance to escape: The lesser binding energy would be exactly compensated for by the greater thermal energy.