GPL3 for the core and default plugins.

No windows: It's normal, because the file manager doesn't support it, see catchcopy project for more information. Ask to the developers of your file browser to support it.
Windows: It should work, uninstall all other copier (included Ultracopier 0.2 if you are on 0.3) and restart. If it not work, see: Copy and paste

The differences are into my Comparative of Supercopier/Teracopy/Copy handler/Ultracopier, into the version 0.3 you can have interface of Teracopy to do easy transition.

See How translate in your language or correct it, into the wiki

Ultracopier do intelligent reorganization of transfer to do to optimize the performance. It's why it not follow exactly the transfer list. See the General informations for more informations.

In my test case yes. But some users report same speed in different conditions. You can see the speed here: Speed comparison. But the most worked features are: data security, software features.

No because it's lot of time to do it (more than time to do all version for windows + mac). Can have binary incompatibility. All the linux prefer do them-self their own package from source.