1) That's it, the 3D tree models are sold separately. Unfortunately, we could not include a full library in the Pro package, without increasing the current price.
Anyway, we are working to include in the next updates a basic set of plants (basically grass and flowers). The models will be included ready-to-use, so our customers can create basic distributions with a single click.
2) Please, upload a test scene to check it. The billboards seems to work correctly here, using the default settings.
3) Forest includes a custom Mental Ray specifically designed for billboards. It improves significatively the render performance. The shader is enabled by default in "Forest Material" (the suggested material for billboards).
For other renderers, an useful tip is to disable the filtering in the opacity map. The "Material Optimizer" (in the Material rollout) applies automatically all these tricks.