"If Sakura would find out Naruto's concerns: that he never had parents or friends... she would be much nicer to him and would probably stop being fury towards him"
She did, she found out during the Rescue Gaara arc, and she also became nice and more concern towards him after that (though she had been on that path since her introduction)...until kishi got new editors.
"I don't understand why Kishimoto let Naruto hide 'this' towards Team-7. Why are Iruka and Gaara the only one ? in Part 1 Sasuke never tried to understand Naruto's past and he only acknowledged him because of his improvement. Sakura also never understood his past and only developed a friendship with him because of his ambitions that surprised her + a lot time spending together."
True, but both of them did learn about Kurama in part two; Sasuke either by being told or figuring it out and Sakura like i said Naruto told her what she was also guessing as well. Though I agree that it would have been better for that to have happened some point in part one.