There is no "type" of reverse engineering involved lol not even CLOSE, look up the term before you use it

It's basicly just a window that is always active untill the engine is launched, when the engine launches it retrieves the handle of the startup window I created and terminates it in the engine window procedure overwritten with on_message.

Basicly a while loop which keeps calling sendMessage(blabalblaa) to the engine window, and inside on_message, when it catches it once, it terminates the visible window. So no I don't see anything illegal here, it's the same as launching an internet explorer when you launch the game and the internet explorer window hides the engine startup window, and when the engine launches, you could terminate the internet explorer window with FindWindow function from WINAPI