Logos (Game Designer) here. Thank you so much for the extensive review! Both for the very kind words, and even more for the construcitve criticism!
M to toggle Music - E to toggle Effects - O to get free stars for the jam (on level selection repeatedly and changing screens to see it refresh)
But even though these feature are in, if you missed them it is still our fault for not conveing this information more clearly.
As for the grid opacity you are probably right, the art department is definitely what we are lacking the most.
Thank you again, I really don't know how to express my gratitude!