1. If I only want to import one single folder for music (only want to play FLAC with JRiver and only want to see Folders with FLAC-Albums), how do I manage do delete the rest already imported?
2. Or ist it easier to delete all (how??) and start a new manual import?
As your new, number two is probably easier and the better option for you. If your library were more mature, filtering the files contained within the library to show only those you would want to delete, and removing them from the library would be the preferred option. I mention this so you know you have this option if you need it in the future.
To clear the library, click "
File > Library > Clear Library"
When the library is cleared, click on "
Tools > Import > Import a single folder"
3. Where do I find the "library" in the JRiver program folder? (Or am I not able do see/edit it at all?)
The library folder is located in your Windows 'user' folder, under Appdata\Roaming. Though it is not advised nor supported for users to go poking around in there unless instructed to do so.
4. If I delete entries in the JRiver library (how?), I don't delete the physical music file, right?
Correct. Though when you press the delete button, you have the option to delete the physical file if that's what you actually want to do.
5. Can I find any information to all this in the WIKI somewhere?
Am not sure. Did you try searching?