Energy in chemical reactions
Magnesium cannot give one or two electrons to the chloride ion. Because Chloride ions have the maximum number of 8 electrons in their external shells. There is no possibility for a new electron to stay on a Chloride ion. And the Chloride ion will be reluctant to accept or to loose one of its electrons, because it has the ideal conformation (8 outer electrons). So there are no "multiple ways" of reacting, as you imagine. The only thing that may happen is for magnesium to loose two electrons per atom, because at the end of this departure, it will have 8 outer electrons, and this seems to be to ultimate goal for all atoms. These electrons will be caught by $H^+$ ions, which are transformed into $H$ atoms and then into $H_2$ molecules.