Is it permissible for one to pray the Night Vigil (Salat al-Qiyam) using recited tapes, given one's weak memorization and desire for prolongation and contemplation?

If you mean to dispense with your recitation in prayer by listening to a tape, then this is incorrect. And if you mean listening and reciting along with the tape, then this contradicts the contemplation of prayer. You can recite from the Mushaf (Qur'an) while standing if you do not memorize the long surahs, though it is better to recite from your memory. You can also recite several short surahs in a single rak'ah to prolong it. As for a boy leading the prayer, the preponderant opinion is that it is permissible if he is more knowledgeable in the Qur'an.