Is the prayer valid if the worshiper limits the Tashahhud to the two Shahadas and sending blessings upon the Prophet (peace be upon him) and then concludes the prayer?

The preponderant opinion is that the final Tashahhud (definition) is a pillar, and the prayer becomes invalid by intentionally or forgetfully omitting it. The minimum acceptable wording for the Tashahhud is: "All greetings [Tahiyyat] are for Allah; peace be upon you, O Prophet, and Allah’s mercy and blessings; peace be upon us and upon the righteous servants of Allah; I bear witness that there is no god but Allah, and that Muhammad is the Messenger of Allah." The minimum for sending blessings upon the Prophet is: "O Allah, bless Muhammad."