You probably figured it out by now, but anyway … 
You may try to install back Trebuchet within an ADB root shell, with pm install --user 0 /system/product/priv-app/TrebuchetQuickStep/TrebuchetQuickStep.apk
Package name is com.android.launcher3, to find the install source from a running /e/ device you can issue pm list packages -f com.android.launcher3.
While your device had no Trebuchet installed, did you try to install the full version from LineageOS ?
I think it may carry out the necessary libraries for the navigation bar, along with the “launcher” intent …