Is the prayer valid if the one led in prayer (ma'mum) pronounces the taslim inadvertently with the Imam, when he was previously deficient by one rak'ah, and then he stood up and completed the rak'ah and performed the prostrations of forgetfulness (sujud al-sahw) and pronounced the taslim again? And should the takbir be for the rak'ah or takbirat al-ihram (the opening takbir)?

Your prayer is valid. Prostration of forgetfulness (Sujud al-Sahw) before the salutations (taslim) is the view of the Shafi'i school. According to the Hanbalis, its place is before the salutations, except in two cases: if one gives the salutation before completing the prayer, and if the Imam builds upon his dominant assumption. This is by way of recommendation. According to the Malikis, if someone gives the salutation believing their prayer to be complete, they should build upon it if the time is short and they have not left the mosque, by saying the opening takbir (takbirat al-ihram). If they omit the takbir, their prayer is not invalidated as long as they intended to complete it. Therefore, it is requested from someone who gives the salutation out of forgetfulness before completing the prayer to return to it with a takbir other than the takbir of transition.