NCL Composer core is available through LGPLv3 (GNU Lesser General Public License).
Each plug-in can be distributed with its own license. The plug-ins available by default with NCL Composer are available through LGPLv3 (GNU Lesser General Public License).
No. There is no restriction with regards to applications developed with NCL Composer. Though, we really encorage you to share your work :).
NCL Composer uses Qt framework as a multiplataform layer. This way, most platforms supported by Qt can run NCL Composer. We support Windows, Linux, and MacOS officially. Look at downloads page for more information.
No. Though if you think NCL Composer is useful, it is recommended that you contribute back to the community. You can contribute with donations, creating documentation, disseminating our project, or in any other way.
Yes. Look at this tutorial: How to create a plugin to NCL Composer.
For now, you can't. But we are working in it. You can find the current install instructions for Ubuntu/Debian in the download page.