What is the ruling if the Imam says the taslim (concluding salutation) while the worshipper is still reciting the Abrahamic prayer in the last tashahhud; should he say the taslim with him or complete it then say the taslim? And what is the most correct (opinion)?

If the Imam says the salām, and the worshipper (ma'mūm) has already recited the obligatory portion of the Tashahhud and the Salutations upon the Prophet (peace be upon him), then he must say the salām immediately. He should not occupy himself with the sunnah elements of the Ibrāhīmiyya prayer or supplication, unless it is very brief. However, if he has not yet recited the obligatory portion of the Tashahhud, he must complete it and then say the salām, even if he lags behind the Imam. This is because the Tashahhud is obligatory, and remaining in the prayer after the Imam’s salām is a matter of scholarly disagreement. The Shāfi'īs, for example, hold that the congregational prayer (qudwah) ends with the Imam's salām, and it is permissible for the worshipper to engage in supplication before saying the salām.