1- I am interested about the "Screen customization" plugin. I would like to know if it is possible to get a demo version before buying it.

Well, it's very difficult.
We canniot publish it on demo as this plugin as very important effects on screens.
You can have a look at user manual available for download on plugin description page.

Hello,
Thank you for your answer for the first question. For the second one, do you need more explanations ?

Regards

2- I am using your tool to import projects with their specifications. When I enter a new project with its specifications, activities, affected resources, milestones, billing ... I need it to be validated by the customer. In case the billing is not validated, I have to change some fields on the current project's screen. That's why I need to set a project's version linked to these few changes. I would like to know if it possible. We can do that with a product.

No, you cannot do that.
Project has no version.
Only Products and documents have notion of "versioning".

You may :
- add field (or use existing) to define version : but you won't be able to "view" previous version.
- copy project to define new version (then close old one)

Thank you very much