hyacinth.88“when i I need to say "she" i I say "he" having slip of tongue, does this ever happen to even to native speakers??”
No. Very rarely, if ever.
hyacinth.88“instead "tell me when he is leaving" i I may say "tell me when is he leaving" i I wonder if it makes a big problem and native speakers do such things.”
Native speakers occasionally use the "wrong" word order in these situations, especially in very long sentences, but I don't recommend imitating that practice. The "wrong" way is understandable, but in most cases it marks the speaker as a foreigner. Practice saying it the right way for shorter sentences, and you will gradually say it right automatically.
CJ