Hi! Thanks so much for completing the game. I hope you enjoyed it.
The text after the final boss is the end of the game. There's only one ending. I could probably make this a little smoother and more clear than just kicking the player straight back to the title screen, so that it doesn't look like a bug. I'll give that some thought.
There's nothing beyond that locked door, and there is no way to open it. When I first designed that level on paper, the layout and events were a little different and you would have needed to explore that area. As I was working on it, I changed some things around and just ended up not making use of that space. I left the door there just for fun, pretty much.