What is the ruling regarding someone who prayed the Shaf' (even-numbered rak'ahs), then stood for the Witr (odd-numbered rak'ahs) without reciting the Tashahhud and Taslim, then completed the Witr and performed the prostration of forgetfulness (Sujud al-Sahw)? Is his prayer valid?

Your standing up from the second rak'ah in Witr without a tashahhud (definition) and salam is correct, and there is no need for prostration of forgetfulness (sujud (definition) al-sahw), because the Prophet, peace and blessings be upon him, used to pray Witr with three rak'ahs, only sitting in the last of them. It is also permissible to pray more than two rak'ahs in Witr.