Is it obligatory to repeat the Fajr and Isha prayers after having doubted whether what was found on the thigh was semen, subsequent to performing ghusl, sleeping, and offering the two prayers?

If you perform ghusl before the 'Isha prayer and then pray, your prayer is valid. As for the trace you saw, if you doubt whether it occurred before or after the Fajr prayer, then act as if it occurred after, and thus your Fajr prayer is valid. However, if you are certain that the trace occurred before the Fajr prayer, and then you doubt whether it is semen (manī) or pre-seminal fluid (madhī), you have the choice: you can consider it semen, perform ghusl, and repeat the Fajr prayer, or consider it pre-seminal fluid, in which case your prayer is valid. This is the view of the Shafi'i school and the fatwa acted upon.