When I scale a layer down to 10% and then scale it back to its original size, it becomes blurry: it loses most of the information in the scaling down step. Is there a way to avoid this?
I would like to play around the sizes of the layers (sizing down & up) until I got all of them as I want to be, and would like GIMP to keep the source images untouched and only actually scale it down when exporting the whole image.
Is this thing possible with another software? Alternatively, is there a way of quickly reloading all source images and scaling them once in the end?
Gimp is a raster image editor; all your image data is in pixels, and most transformations done on these pixels will inevitably cause you to lose data. If you want to keep your original image around while using it scaled, you could simply make a new layer for the downscaled image, while keeping the original layer(s) it's based on in the project, but simply setting it invisible. Or, you could just keep the non-scaled original in a separate project, as original quality backup. It's not Gimp's job to do your WIP management for you.
If you don't want to use backup-layers but are really looking for scalable content, you should look into using vectors instead. Gimp has rudimentary vector support with its Paths tool. It allows you to create shapes and curves which can be converted to a selection, which can then be filled with colours. Text data from fonts can be converted to such paths, and you can export paths themselves as svg files.
As I said, this support is rudimentary. Gimp is not the ideal tool for this kind of thing. Expect to do some manual xml editing if you want those svg files to be anything more than simple black outlines of your shapes. If you really want 100% scalable content, look into actual vector drawing applications, like InkScape.