Is a lone worshipper, if two trustworthy people alert him to a mistake he made, obligated to follow their word, such that his prayer would be invalid if he does not do so and is not certain of his own correctness, or is this ruling specific to alerting the imam by the congregants?

If the Imam concludes his prayer out of forgetfulness and is alerted by the companions, he should rectify the oversight and perform the prostration of forgetfulness (sujūd al-sahw). The alert from those not participating in the prayer is acceptable if their correctness is preponderant, because the Sharia does not differentiate between similar cases. The worshiper, if he forgets, is commanded to seek what is correct, complete what remains, and then perform two prostrations. This indicates the obligation to act upon thengthening of conviction (ghalabat al-ẓann), such as two individuals alerting a solitary worshiper if he is strongly convinced of the correctness of their alert. Scholars have explicitly stated that alerting a solitary worshiper about his forgetfulness is similar to alerting the Imam.