Is it permissible not to observe the sequence when reciting surahs, and to move from one surah to another before completing the first when reciting from memory? Is it permissible to recite while walking? And is there any text mentioning a guaranteed answered supplication with every completion of the Qur’an?

There is no harm in completing the Qur’an without observing the order of the surahs, or starting a surah (definition) before finishing the recitation of the one preceding it, even though it is preferable to complete it in order. Some scholars disliked reciting the Qur’an in the streets, while others permitted it. The hadith (definition) "At every completion, there is an answered supplication" is very weak; rather, Al-Albani described it as fabricated. As for gathering to supplicate after completing the Qur’an, it is recommended because it is a time when supplications are likely to be answered.