Is the prayer of a congregant valid if he says the Takbeerat Al-Ihram while the Imam is saying the Tasleem to end the prayer, and the congregant then completes his prayer individually?

Your prayer is valid, but the attainment of the virtue of congregational prayer differs among the schools of thought. According to the Malikis and those who agree with them, the virtue is only attained by catching a complete rak'ah, based on the Prophet's (peace and blessings be upon him) saying: "Whoever catches one rak'ah of the prayer has caught the prayer." As for the Hanafis and Hanbalis, and this is the correct view among the Shafi'is, one attains the virtue of congregational prayer if one says the takbir before the Imam's first taslim, even if one does not sit with him. However, if you do not say the takbir before the Imam's taslim, then you have not attained the virtue of congregational prayer according to all.