Is my prayer valid if I sat for the Tashahhud in the first rak'ah of the supererogatory prayer, then remembered and stood for the second rak'ah, and performed the prostration of forgetfulness (sujud al-sahw) after the prayer?

The Maliki school and Ibn Taymiyyah hold that the sujud al-ba'di (prostration of forgetfulness performed after the taslim) should be performed after the taslim if there was an addition to the sitting for the tashahhud. Its delay does not matter, as it can be performed whenever the worshiper remembers it, even after a long time. The prayer is not invalidated by its omission at all.