Yes, but the assets are made for cycles and eevee.
It's compatible with eevee but it's an overall bad choice for environmental rendering.
Yes, 5.0 will be reworked for the new particle node system, no worries.
Scatter is the most optimized solution right now, unfortunately working with millions over millions of instanced objects is always tricky. you can learn more about that in our manual.
For the Addon it's GPL
For the models, it's Standard Royalty Free
For the generated renders it's CC0 4.0 (duh)
so in short: do whatever you want but don't resell the biomes assets at your expenses ( you can share everything this with your clients, i have no problem with that )
Yes you can.
There's a built-in handy addon made for drawing instances of objects easily but it's not a true scattering toolset like we expect from other software. It's called "object-scatter".