Had to give up on LibreElec because it just doesn't work.
I installed the latest Raspbian Stretch Lite. Then installed Kodi out of the repo.
At this point when running Kodi it is visible from UPNP all the time. It responds to all discovery requests. LibreElec would only respond after immediate boot / reboot of the Pi. So whatever is going on with UPNP discovery requests, it is definitely an issue related to LibreElec.
As to seeing remotely mounted FS (NFS or Samba), loaded into the library that problem still exists. But it seems to only exist with Roku.
In Roku I can see the Pi server. I can look at the library. I can see all details of the video but when selecting the video to play I get a corrupt file warning or invalid file, etc. etc.
If I use a different UPNP client say on my Android Tablet I CAN select the video and it starts playing. So this is more of a specific Roku issue.
However, since I used Raspbian, I am now able to use the system to mount the remote NFS to a mount point on the Pi. If I then tell Kodi to use that local file system location to load the library everything works perfectly in all clients including Roku. So while it is Roku specific problem it is related to the NFS and or Samba mounting when done within Kodi and not via the file system.
Now the only issue I have remaining is that Raspbian Stretch seems to have issue mounting remote NFS during boot.