Firstly: The ruling on the first Tashahhud is that it is recommended, according to the consensus of scholars from the Ḥanafiys, Mālikiys, Shāfi῾iys, Ath-Thawriy, and Al-Awza῾iy. However, the Ḥanbaliys and Dawūd consider it obligatory not recommended.
Secondly: The principle is that it is not permissible to leave a pillar and engage in something else during its time. Standing (Qiyām) is a pillar, and Tashahhud is recommended. So, whoever stands for the pillar (standing) should carry on and not retreat. They should not return to Tashahhud. They have the option to prostrate for forgetfulness due to omitting the Tashahhud. If they do not prostrate for forgetfulness for forgetting the Tashahhud, their prayer is considered valid, according to the majority of scholars.
Fatwa issued Dr. Khālid Naṣr