I'm a sucker for games with Endless modes, so this is already a big win for me!
The notepad-spell mechanic is so fresh and interesting! You know I tried to make a similar design with my submission, but yours is much more concrete and functional. Using the numpad in specific is a great catch because it's far from the conventional arrow-keys-or-WASD controls we're already used to and forces the player to rethink and look at what he/she is pressing, further supporting the jam's theme.
Understanding how to play a song correctly takes more time than I think it should (you're supposed to read the notes left to right, not top to bottom like the numbers suggest), but the end result is satisfiying.
Using spells in the middle of the heat, however, is pretty confusing, which for me is a great point considering this makes you feel more out of control. Learning how to cast Fireballs consistently so you don't need to focus on that during battle feels like learning how to do a combo in a fighting game so you don't miss it in an actual match, and I loved how the same feeling could be applied to a totally different game.
For the downsides, I don't really think losing notes were necessary, specially because most songs use a wide array of notes and are rendered useless without one of them. You could either make it so songs are harder to play (by hiding their note sheets for some time and requiring players to play from memory, for instance) or make simpler, infallible combat songs that can be used anytime, like a 2-note melee attack song.
For a more technical critique, I also didn't like how the player is locked into LookAt(nearestEnemy) at all times as long as there's one alive. Sometimes the enemy is be all the way on the other side of the level, and the strafing motion used when looking at it gets amplified when farther away, giving it a "dragging" feel when walking towards certain directions (which is a direct effect of the LookAt approach). It'd be pretty simple to just limit this feature to enemies within a certain Vector3.distance range from the player :)
In presentation terms, the game's just plain and simple adorable. Great use of colors and contrast. Animations, particles and effects were so abundant that there was really no need to make actual, rigged animations on models.