What is required of the congregants to do after discovering a fifth rak'ah was added to the Dhuhr prayer performed in congregation, due to the Imam's inadvertent sitting for the first tashahhud in the third rak'ah, and the error not being rectified by performing the prostrations of forgetfulness (sujood al-sahw)?

What you did by counting the second doubted rak'ah, then sitting for the tashahhud and completing the prayer, is in accordance with the Sunnah. However, you should have performed the prostration of forgetfulness (sujood as-sahw) before the salutation (tasleem), because the Prophet, peace and blessings be upon him, commanded anyone who doubts in their prayer to build upon the lesser, certain number, then prostrate twice before the tasleem. Since you omitted the prostration of forgetfulness and a long time has passed, it is no longer required of you, and nothing is binding upon you.