In Germany, prayer time apps often differ, and even masajid in the same area have different times. Should we follow the nearest masjid? We can’t calculate/research every prayer time ourselves, and most local masajid are run by people who follow Ahlul Bid’ah. How should we approach this situation?

‎You may not pray until you are sure, or strongly believe, the time has entered. If a masjid follows a known, trusted calendar, praying with it is fine whatever its people's other mistakes, unless it is proven to pray early. When timetables differ, taking the later time is safer, and delaying a few minutes does no harm.