Is the congregant's waiting for the imam - after the opening Takbir - while standing, and the imam is about to rise from prostration, considered preceding the imam, and what is the ruling on the prayer?

What happened is not considered preceding the Imam, as you were not following him in prostration and then you rose before him. It is better to join the Imam in the state you find him in. There is no need to repeat the prayer, as it is valid and nothing occurred to invalidate it.