Is my prayer valid after performing ghusl from janabah, wudu, and praying, then remembering that I did not clip my nails and doubting whether water reached beneath them, so I washed them and prayed again? And was that a waswasa (whisper from Satan)?

The first ghusl was valid, and there is no need to repeat it or the prayer. This is because doubt about an act of worship after completing it does not affect its validity. It is advisable to strive against obsessive thoughts (waswas) and to disregard them.