Hi Rene,
Thanks for describing your system and the issues, it should greatly assist the Support devs (@mike and @vova) to work out 1 and 2. They will be in touch shortly as they read this Support forum every day.
I can help with 3 and 4.
There are three levels of metadata in Roon. Firstly the file tags in your FLAC files, secondly any tags or edits you make in Roon and thirdly metadata provided by third party providers when an album is identified (Allmusic). You can tell Roon what metadata to prefer for an album or batch of albums.
This FAQ will guide you as to how to shift files and best preserve your edits, cover art etc. The links from it will tell you where the Roon database is located, but it may be that you are not shifting the database if you are just moving your music files.
Playlists in Roon don’t currently support cover display but it is coming. Brian2 is currently working on playlist design and has requested user input here.
Hope the above helps, the Support devs should be along soon regarding the other issues.