Hello there !
I can give you a few addons that I found quite useful and time-saving.
- https://github.com/remorse107/rmsmartshape2d lets you create curves, filled or not, and attach a material containing different textures for the Top, Right, Bottom, Left, Inner Left and Right parts, giving you an easy way to create terrain like they did in Rayman or like Ferr2D Terrain for Unity. The addon hasn't got any update for a year and I'm currently working on a similar addon with an easier architecture and some options, like the ability of switching any edge's texture so you can have variants, but that will take time to get released
- Zylann's https://github.com/Zylann/godot_heightmap_plugin will let you create terrain through generated maps and a few setting files
- Zylann's https://github.com/Zylann/godot_scatter_plugin will let you place any mesh on the terrain and that works pretty well, it is compatible with a heightmap aswell and has interesting features. I recommend watching his tutorials about his addons, they show you everything you need
I agree we could have more advanced terrain editor tools, but I have to admit I'm not sure it would be fit for everyone as there are many ways to create terrains from a game to another and we would need to have a more general editor. You are also free to code your own tools to fit your needs by using any accessible class and Node through EditorPlugin, following tutorials like: https://docs.godotengine.org/en/3.2/tutorials/plugins/editor/making_plugins.html
For the rest of your questions, better have answers from a developer !