Turbo Trees is primarily focused on filling your scene with customized billboard trees. There is nothing included related to terrains.

Sure! The example images and render were done with Cycles, but the shader settings work with Eevee and can be tweaked (see page 9 of documentation).

Feel free to use what works best!
I personally try to only use solutions that aren’t overly complex or unnecessarily heavy. And since this is just a geonodes modifier + shader you can freely peek inside and change anything. The geonodes modifier has been organized and labelled neatly. I created this for my own project as I didn’t feel there was an affordable and drop in solution.

Yes! Just be sure to toggle off the align to camera option.

Yes! I will be adding more as time goes on but the setup is fairly easy
to get going. I will add some documentation on adding your own tree
textures shortly post release.

Ensure that the weight paint group is called ‘custom’ . The modifier
will only read groups called custom.

Ensure that Density Factor is not 0, alternatively ensure that if you
have a max height set, the value isn’t too small.

You will need to increase the ‘Transparent Bounces’ under Light Paths in the render options.