Is it incumbent upon one who prays Tarawih and wishes to delay Witr to recite the Tashahhud and make the Taslim with the Imam, then pray a single rak'ah and make the Taslim, and should his Tashahhud be complete or not?

There is no harm in a person praying with the Imam. When the Imam says the salām, the worshipper should stand up and perform one rak'ah to make his prayer an even number, then pray the witr at the end of the night. If the Imam prays witr and sits for the tashahhud, the worshipper must recite the tashahhud, as it is a pillar of the prayer. It is best to recite the complete tashahhud and send blessings upon the Prophet (peace and blessings be upon him), then seek refuge with Allah from the torment of the grave, if the Imam's sitting time allows it. If he only recites the first tashahhud, there is no harm in that, and his prayer is valid. If the Imam says the salām and the worshipper stands up before completing the prayer or du'a, there is no harm in that either.