It's quite quite active with monthly releases!

See here on ways you can contribute to Armory.
Thank you for helping Armory grow!

Absolutely!
The Armory Engine is licensed under zlib/libpng, which entitles you to sell your own game commercially if you wish to.
Note: Be aware that the Armory add-on is however licensed under its own licensing - Blender GPL licensing.

Please follow basic troubleshooting steps from here.
If error(s) still persist, please open an ticket or ask for help at any of the Armory community platforms.

The Armory manual has a full list of logic node references.
The references contain detailed descriptions of what each logic node is capable of and what features it comprises of.

To avoid bias, you"ll need to conclude that on your own.
Here is a list of pros and cons of both game-engines that is maintained by the community.
Feel free to open a pull request if you feel additional information could be added to the list.