Hey Hazem, how are you, my friend? I hope you are doing just fine!
''Thousands of personnel files were carefully checked. The search proved beyond any question that no significant difference existed in native intelligence. What's more, differences in education did not explain the difference in selling success. The difference in the very successful and the very unsuccessful finally reduced to differences in attitudes or difference in thought-management''.
1) ''beyond any question'' = in this case, the word ''beyond'' is used to mean ''surpassing'' or even ''exceeding''. So, you may get this idea: ''...beyond any question...'' = ...exceeding/surpassing any question... = ...undoubtedly/without a doubt/certainly...
2) ''reduced to'' = the idiom ''reduce to'' might mean ''decrease/bring something to a(nother)level''. So, the idea conveyed is that the differences in the very successful and the very unsuccessful finally ''changed to'' or ''turned out to be'' those in attitudes or thought-management.
Did you get the idea?
Hope that helps!!!