You can install apps with both Sidequest or ADB. However, for launching the app you need to perform for exemple :
adb shell monkey -p com.google.android.apps.youtube.vr -c android.intent.category.LAUNCHER 1
It launches but it don't work properly. I'm sure there are a workaround to get the app working but you need some Android programming and VR knowlegdes (which i don't have)