Blender 5.1 changed its underlying python version which required all included python modules for TrueSKY and TrueTERRAIN to be re-built and therefore not backwards compatible. You do have access to previous files that did work in Blender 4.5, though some minor optimizations in TrueSKY are not included in them.

Kinda, you can use any of the products here royalty free. This means you can create images, animations, films, whatever from these tools, and then use those outputs in whichever way you want. Like selling your renders, or animations. You CANNOT redistribute any of the content, including object files, materials, nodegroups.

Yes, we provide minor and bug fix updates, as well as many feature updates during major versions releases. TrueTERRAIN 5 has had many new features added between 5.0 and 5.1 for free to existing users. We only charge for Major updates which usually require a lot of work over a long time.