Is it possible to complete memorizing the Qur’an in two to three months, given that six parts (Juz') have been memorized, half of which are well-retained and the other half are almost forgotten? And is it better to review the forgotten parts or to start memorizing new ones?

The possibility of completing Quranic memorization in a short period depends on diligence, frequent revision, and the ability to memorize, after Allah's enabling grace. If these are not present, then some will be memorized, and the rest pursued later, while maintaining what has been memorized. As for what is most prioritized, it is to combine revision and new memorization if possible. Otherwise, revision takes precedence to solidify the memorization, for what is already possessed is more important than seeking what is not yet found.