@roslof, Don't know if you're still having problems with lr-scummvm but perhaps telling you about my experience will help.
I was succesfully using lr-scummvm on a RPi 4B RetroPie build that used RetroArch version 1.8.8. I recently set up a second Pi 4B with a completely fresh RetroPie install using the latest RetroPie image available. The RetroArch version here was 1.9.x (I forget exactly which) but I found that lr-scummvm would crash when I loaded up any game in just the way you described in your original post.
My thought was that, although it was probably ultimately a fault with the experimental nature of the lr-scummvm core, the crux of the problem was that the lr-scummvm core was not compatible with RetroArch v1.9.x as it seemed to be with 1.8.8.
So, without wanting to completely reinstall my whole system, I installed the older RetroArch v1.8.8, overwriting the one that came bundled with the image. I didn't need to change anything else and, happily, I was able to get my favourite LucasArts games up and running straight away.
Now, obviously, downgrading Retroarch can be a bit risky so if you want to try it you do so at your own risk. Unfortunately I can't tell you how to do from the command line as I couldn't find that out. But I managed it by backing up the ~/RetroPie-Setup folder (by adding ".bak" to the end of the folder name) and then copying the folder from my older system to the new one. Then I ran the retropie_setup.sh script that I just copied and went to Manage Packages | Manage Core Packages | Retroarch | Install From Source. 10 minutes later, RA 1.8.8 was on my system and lr-scummvm worked beautifully.