Thanks for your feedback, there certainly are areas to improve the game.
For music you are right, there's only one track. And I wont deny that boss-specific themes would have been ideal.
Overall whilst the game is simple (at least to me) there does seems to be a miscommunication on certain gameplay elements. Simply put the tutorial is lacking and could do with some re-scripting to be more interactive. You are not the first person to favour left click over right click.
In short yes, there are some things you missed:
On the top left besides your health, that is your spin, the more 'full' the coloured ring the more spin you have, the middle part also turns blue when full. As long as there is still a coloured ring in this 'meter' you should be able to left click.
As for the spin attack (right click), this is faster the more full the meter is and all deflected attacks will always aim at the cursor from the moment the shackle hits them, so point that cursor at the boss and just 'spin to win'.
Regardless I greatly appreciate the detailed summary of your experiences, difficulties and overall views of the game.
And honestly the fact that the game can be won even with only using the left click is both surprising and impressive. I shows to me that the game can be played either way, and which is more fun that's up to the player. So in short I'll take it as part failure and part success and use it as a learning experience.