Does the Imam's prayer become void if he adds a prostration based on the alert of a follower who thought he had forgotten a rak'ah, while the Imam is certain he had not forgotten anything?

The Imam does not pay attention to the recitation of 'Subhan Allah' by the congregant unless he becomes predominantly certain of its truthfulness. In such a case, he acts upon his predominant certainty, not merely the congregant's recitation.

If the Imam doubts because of the congregant's 'Subhan Allah' but is certain of the number of rak'ahs, and he doubts regarding the two prostrations, he should perform them and then perform the prostrations of forgetfulness (sujood as-sahw). This is the correct approach. He should not pay attention to the congregant's statement about missing an entire rak'ah as long as he (the Imam) is certain.