have you tried this install script, this is beta 0.009 but I it installed perfect for me on OSMC KODI running the latest official OSMC (KODI 17.6)
You might get an error about a joytokey python script missing, but that shouldn’t cause any issues as all that gets installed with RetroPie
I did try updating to KODI 18 but had no end of video issues, so went back to OSMC KODI 17.6 and all the video problems went away. But, it does clearly state KODI 18 is still in ALPHA but gave it a try
Try installing the latest RetroPie script, via the RetroPie config, or run the RetroSMC script again and it should automatically take you there. Then, do a rebuild ALL. It WILL take a loooong time. Overnight is better but should update ALL RetroPie to Debian Stretch
It might be easier to do a full fresh install of OSMC and once that is running, THEN install the beat 0.009 RetroSMC script. Its a major pain I know, but if you’ve been messing for a long time, sometimes a fresh install can fix everything
I run OSMC off a USB hard drive, not that should make a difference, using the ‘install to USB’ option when initially starting OSMC from a fresh install