With much research by olly (whom I thank), it seems likely that my problem is with administrator or disk permissions blocking normal game execution and causing those chaotic freezes.
We tried a lot of things that didn't work, so I settled on a non-ideal solution but one that works : I establish a Teamviewer connection between the virtualbox windows 10 and my real windows 10, I mute the sound of the virtualbox version, I launch the game... and no more freezes !
So if anyone had the same problem as me, they can try this.