Maybe buffer size isn't large enough for HD audio. Try this at /storage/.kodi/userdata/advancedsettings.xml:
Code<advancedsettings version="1.0"> <cache> <memorysize>2000000000</memorysize> <!-- 2GB buffer size --> </cache> </advancedsettings>
I would not recommend this
The files need to be in the same location as that's how the library tracks file changes.
You can use path substitution but it's never really a good solution Path substitution - Official Kodi Wiki
it it never started after enabling it would have been interesting to see why it didn't start
Just to make sure I understand correctly, there is no option for the audio/video sync. And.....there is no option to use a codec licenses.
Correct
You are free to downgrade if you feel like you need to 
It's probably a better idea if you just describe the problem you are experiencing and we can tell you how to fix them. Otherwise everyone is just guessing
the root partition "/" shows 0 free space because it is a loopback (because of the squashfs filesystem). Either you need to filter it or ignore it
No idea, I dual boot with a grub installation but it was done manually by me
Would you be able to upload a sample video file that produces this problem?
I'm assuming you mean on an RPi4? AFAIK that isn't supported yet and you shouldn't be using the 4k60 option
It's not really plug and play as it's been tuned for LibreELEC.
You can see how the sharing details work here though LibreELEC.tv/smb.conf at master · LibreELEC/LibreELEC.tv · GitHub
Why don't you try disabling the internal BT with a dtoverlay?
I have no idea what you are talking about but if you think it's a bug you should report it to the kodi-pvr/pvr.iptvsimple repo
Chrome add-on : any plan to add it to version 10 ?
Until now, it's the best way ( and the only always working" to have Netflix and Amazon , via browser, on Libreelec-Kodi.
Hope it will be ported
It's unlikely. If that's something you depend on I would suggest dual booting or looking into using Kodi with netflix and amazon
Does it backup addons because it only mentions foe kodi settings, libreelec settings and the database and not addon data and addons....
The entire ~/.kodi directory is backed up so yes
You likely don't have an internet connection when Kodi is started. You can try using the option in the LibreELEC settings to delay startup until a network connection is achieved.
Use the backup option in the LibreELEC settings add-on
