I think you can achieve this by modifying the retroarch service.
I’m not sure on how exactly to do it, but first try to stop and disable the default retroarch service like this:
systemctl stop retroarch
systemctl disable retroarch
With this, retroarch should stop launching.
Then, I think that OpenELEC can let you define your own user space systemd services. Copy the one we use, and add the arguments you need to force the core and the rom, and enable that service.
Let find some documentation about systemd and OpenELEC custom systemd services.