Must I follow the unified Fajr prayer time in most mosques in a European country, which appears to contradict the Prophet's hadiths, or should I follow the Prophet's hadiths and the scholars who oppose these mosques?

Prayer is not permissible unless one is certain or has a strong presumption that its time has begun. If you predominantly believe that the few scholars in your country are correct, then do not pray according to the many whose timings you doubt.