It was pretty easy. The perfect level of simplicity for an introductory level.
About a minute.
I lovedddddd it. Being able to control the character during a slow zoom-in was really cool. It set the mood well.
Yes I think I would be able to. I think experiencing the game without being guided by instructions could make the experience feel a lot more profound and immersive.
The lights which slowly rise up once you collect them are good enough hints for me. I would not feel the need for more.
There doesn't seem to be any narrative beyond "collect the gems in a fantasy world". I think cave paintings on the wall which slowly become revealed as you light up the maze could add a really cool yet subtle dimension of narrative to the story. It would be really challenging to implement but would add another layer to your game for sure.