@Cupcakegirlpower
1. I believe that decision should be up to you. Some people like songfic while others don't. I just think that all depends on what is contained w/in the chapter as a whole, especially if the song takes away the main impact/purpose of a scene. But songs can also help embed the purpose of a specific scene, just like how it does in movies and TV shows. So, as a whole, I'd say to do the songs sparingly if it'll help convey a scene.
2. Again, I believe it just depends what the chapter is supposed to contain. Long chapters are good, but they can't go on for too long cuz then it may make the reader less invested, or they'll leave after reading a bit of it, and then completely forget about finishing the chapter. (I make that mistake all the time, actually).
3. While I would love more POVs of Eugene and Lance, the best would probably be to do what happens in Heroes of Olympus, where 1 chapter is the name of the character's POV, and then it swaps between the 3 of Rapunzel, Varian, and Cassandra (but I would say to do the same order, like 1-2-3 continously, and not do 1-3-1-2-2).
4. I would only suggest more descriptive details of the world your characters live in, as well as their emotions a bit more. But don't forget that you should be having fun writing this. You don't haveta use any of the criticism here, as it is ultimately your decision. You write the story YOU want to write, not what others expect or desire. Hope these suggestions may help guide your writing, bestie! ♡♡♡♡ (If you'd like, of course)