finding a route to form mode and add it local tasks
I could find a solution with an handy module: https://www.drupal.org/project/form_mode_control

After installation, you can reach any form by using :

 www.your-site.com/node/add/my_content_type?display=machine_name_of_your_form


or

www.your-site.com/node/{node}/edit?display=machine_name_of_your_form


More then this, this module allow you to set permission to access each form mode.

I am still looking for how to add those forms as local task... but the main thing is done 

My objective is: if I go to node/add, or node/{node}/edit, I would like to have the default form displayed but also, all the form mode accessible as Tabs (local task)