Hi Steffen,
You can change a repository from link to copy, that is fine. After that there is no longer a live link between the repository and the styles in the project. Basically, the repository is used once as a template. Changes made in the repository after that will no longer be reflected in the project.
* go to "Edit Styles..." and change the "Normal" style and/or some other styles
* save everything as another repository
* switch between the old and the new repository and their styles as needed
That is also possible. You just need to make sure that you really switch when you make the change. When asked to delete the existing styles in the project you should do that. Otherwise the new link will give you both styles, with different names, and that will basically be a mess...
* if the repository is set to "Link the styles of repository", there can be no style differences between repository and project because even if I made changes before, as soon as I switch back to "Link the styles of repository" these are undone.
* editing styles in the project does never affect the data in the repository
That's correct. If you apply the style repository styles they will replace all styles of the same name that exist in the project. You can only edit the styles in the project if they are not linked to the repository, and they never go back into the repository.