Please un-zip the archive and open the .blend project.

If you have issues in Blender 3.1 with real-time previews, then just enable playback and you will see changes instantly.

Yes you can, but the intended shown uses on the demo video are different in regards to the partial smoke particle effect. The lightweightness of the 2D lights is reduced render times and easier previews. 

License is Royalty Free. You can use this commercially and non-commercially, as long as you don't violate any of the following: 

- You may not resell, redistribute, or repackage the purchased product, or seperate parts of the product, without explicit permission from the original creator

-  You may not use the purchased product in a logo, watermark, or trademark of any kind with the exception of: shader, material, and texture products are exempt as the product is used in a secondary manner.

Provided you know the basics of Blender and have watched the tutorial for this pack, you shouldn't have any issues using the assets.