If user pressed back button during application startup. Application becomes unresponsive.

All you need to check this: run any qtquickcontrols2 example on android and press android hardware back button(do it several times to make sure app received event).

After that initial scene shown, but not updated. And user can't interact with it.

Now if you somehow put app to background(with home button or switch to other application) and back to qt app you'll see just black screen without scene.

The only way to kill app is to close it with android applications menu or swipe it with application switch functionality.

I also want to mention that QApplication's thread stays alive. If I put timer and trigger some printing function I'll see it in android log.