Is reading a complete recitation (khatmah) to review memorization without contemplation, and reading another complete recitation for contemplation during supererogatory prayers, excluding the two rak'ahs of Fajr and and Maghrib, considered an innovation (bid'ah) or a commendable act?

There is no harm in dedicating one recitation (khatmah) for contemplation and another for quick reading. Reading the Qur'an and frequently completing its recitation is a good deed. Scholars have differed on which is better: frequent reading even if it lacks contemplation, or contemplation even if the reading is infrequent. There are two opinions on this. The correct view is that reading with contemplation yields a greater reward. A difference in preference between the two was reported from Imam Ahmad, and the Companions and their successors preferred reading with contemplation.