4.2 LTS and newer. I test on 4.2 LTS and 5.2 on Windows. It is a plain Python extension, but I have not tested it on Mac or Linux myself.

Yes, updates are free. Every new version waits in your Superhive account. Install the new zip the same way, your atlases stay in your .blend files.

Yes. Your models and textures are yours. The GPL licence covers only the add-on's code.

Yes. The result is a normal texture and normal UVs. Import the atlas without compression, and either turn mipmaps off for it or keep the swatch margin at 2 px or more (that is the default). Set filtering to point if you want hard pixel edges.

It writes the UVs of the faces you colour, in the active UV map. If you want to keep an older unwrap, add a second UV map first and make it active.

Mipmaps blend neighbouring swatches at a distance. Keep the Margin at 2 px or more (Classic controls and more settings, step Map, Settings), or turn mipmaps off for the atlas.