Leveling drives me nuts.

I have a finished project of 6 months for four people, and want to instert a task for one specific person. When I insert it and level, instead of just moving the rest (they also have lower priority) 5 days after, my project turns into a mess.

Could someone tell me what am I doing wrong? The project I want to insert is the only manually scheduled one, the moment I turn it into auto scheduled and level hell starts.
Here is the file.

https://1fichier.com/?ao6hdk5w0v

Thank you very much.