Thank you!
2. Yes, you can sell them, and if you don't want to, you don't even have to mention which engine it was made with. There is no splash screen either.
3. Yes, the first one will be for Gmonenstein 3D. But the engine is still very new, so for now, there is only a manual.
4. It is about as hard as building it in Unity. So it is definitely doable.