Complex sentence: Many late-medieval countries of Europe were officially elective monarchies, but the same family had held the throne already even centuries, and that hybrid situation should be described as pseudo-elective, virtually hereditary monarchies, the succession system being in slow transition.
Split into simpler sentences: Many late-medieval countries of Europe were officially elective monarchies, but the same family had held the throne for centuries. Such situations may be more aptly described as pseudo-elective, virtually hereditary monarchies, with the succession system being in slow transition.