What is the ruling on the prayer of someone who said the takbir while the imam was descending to prostrate, then completed his prostration alone after the imam had raised his head? And what if he did not prostrate in this situation?

If you catch the Imam rising from prostration, do not prostrate but rise with him. What you did by prostrating after the Imam had risen is incorrect, and what is obligatory is to follow him in rising. Your prayer is valid because the increase occurred due to ignorance of the ruling, and you must abandon these whispers (waswas).