[Milhouse]

Q: Can we simply drop a build from i.e. https://libreelec.tv/downloads_new/raspberry-pi-3-3/ to switch back to the mainline version?

Yes, as the database are still the same version.

Will this retain the installed addons and configuration, i.e. Amazon Prime, inputstream.adaptive, etc. or does this require some extra work - or perhaps even custom build?

You'll need to install inputstream.adaptive as it's not included as standard in release builds (whereas it comes bundled with these test builds). Same goes for all of the PVR addons, and also vfs.rar/vfs.sftp/vfs.libarchive.

How about adding 1-2 sentences in that regard to the first post?

I doubt anybody would read it.