Is it better to complete Witr with the Imam in Tarawih prayer to obtain the reward of Qiyam al-Layl, or not to pray Witr with him? And is it permissible to intend the post-isha sunnah with the Imam during the first two rak'ahs of Tarawih if there isn't enough time for the regular sunnah prayer?

It is better for one who prays Tarawih with the Imam to pray Witr with him, due to the Hadith: "Indeed, whoever prays with the Imam until he departs, it will be recorded for him as having spent the entire night in prayer." As for whoever wishes to pray Witr at the end of the night, let him pray one Rak'ah after the Imam's Witr to make his Witr even, for the Prophet, peace and blessings be upon him, forbade two Witrs in one night. As for the post-Isha regular supererogatory prayer (ratibah), there is no objection to praying it after Tarawih if it has not been prayed before, and it is permissible to intend it behind the Imam while he is praying Tarawih.