chocco No problem! Does your source image have any partial transparency in it? That's the only thing I can think of that could be causing a difference. Any pixels with partial transparency are not supported yet and will be set as opaque when loaded into the Voxel Editor. Otherwise, everything is imported one for one directly from the source image.
Currently there are a couple of options for creating a sky from an image. One method, if you're wanting a flat backdrop behind the scene, is to split it up into tiles by importing sections of the image into the Voxel Editor (as shown in the tutorial), then placing them into the map individually. Another option is to use the "Panorama Sky" background setting on the Map Properties tab. If you have a traditional skybox image, there's a convenient tool here that will convert it into the proper panorama format:
https://danilw.github.io/GLSL-howto/cubemap_to_panorama_js/cubemap_to_panorama.html
RPG in a Box is primarily designed around creating 3D tiles, objects, and characters and building a map from those, however I expect there will be more options for static backgrounds once I move to Godot 4. I also plan to gradually provide more flexibility around using images, sprites, and 2D support in general.
Regarding question 4, you can use the "Camera Override" setting on the Map Properties tab. This camera type will be used when loading that particular map. In the next release, I'm also adding some camera preset functionality that will allow further customization of the camera settings between different maps, beyond just the main camera type.