frakkin64 thanks for your reply.
I'm happy to report that I made it work based on your suggestion to use the bridge/tethering option.
i initially went down the route of playing with iptables because that was what i'd done before but i couldn't make it work. I copied the contents of my sd card and reinstalled everything from scratch. Then I tried the commands you shared but i couldn't make the routing work. Funnily enough, though, they do work on raspbian and an older raspberry pi i found. I still believe there may be something in libreelec's implementation that may be preventing this. I made sure ip_forwarding was enabled and tried a few more commands that I found googling, but nothing.
so, anyway, what you suggested works but i have to say it's not really stable. it's not every time i reboot, but more often than not, the raspberry has no connectivity. configuration is kept when you reboot (in the file suggested in the links you shared), but i have to manually disable/enable wifi from the Kodi to gain access back.
i tried to automatically "fix" this by enabling/disabling the wifi via crontab, but it doesn't seem to do anything.
It's not all the time and it only takes one second to fix. I keep this machine running all the time, so it doesn't bother me that much, but anyone looking for a similar setup may find this useful.