[quote]Will it be possible to mark a reference at the begining ? To move the whole installation to show that's the same in the right and in the left ? Maybe it'll be better with a sharp object ?[/quote]

Because you ask me to show the same displacement in the right and in the left, that is why I add the "show displacement" as option and set it as default.
Now, you want [quote]I'm wondering too if it's possible not to see displacement by default.[/quote]???
I am confused!

[quote]But in the first experiment, is it possible to let a trace of the ball after make it goes up (I mean a circle in dot line to see that when we will let it fall, it'll go at the same place) ?
And a moving square to show that the trace is perpendicular to the ground.[/quote]

Could you create a simple drawing show me what you really want, and explain what you want to do/ how you will use it for teaching purpose? Please also indicates where you want to add "protractor"!

As far as I know, for the name of the tabs:
you will need to download ejs xml source and modify the name of the tab,compile it again  to make it appear.
Change the name of the tab is not supported at this time yet.