How can one reconcile commitment to the Sunnah of 12 daily rak'ahs with the hadith, "Whoever prays 4 [rak'ahs] before Dhuhr and 4 [rak'ahs] after Dhuhr, Allah will forbid him from the Fire," and does increasing [the number of rak'ahs] to 14 contradict the hadith that mentions 12 rak'ahs?

There is no contradiction between the hadiths encouraging the prayer of twelve rak'ahs and the hadiths encouraging the prayer of four rak'ahs before Dhuhr and four after it. Both are authentic, and the first hadith (definition) does not prohibit exceeding twelve rak'ahs. Thus, the worshiper would have performed what is mentioned in the first hadith and more, attaining great reward. It is recommended to perform many supererogatory prayers, as they are a reason for Allah's love, elevation of ranks, and expiation of sins.