because people will install your app and then uninstall it. However they may still retain the OTHER developer's app that includes your SDK. This is just how customers behave.

If your flow is blocking on Mondo app being installed - that's fine. This means that the surface area of attack is restricted around your app. That's totally OK.

However - that is a very different positioning than oauth. I would say Oauth will degrade gracefully to your protocol if the endpoint is restricted to another app that must mandatorily be installed on the host device.