First off, I really enjoyed your game. Last night, I just finished completing it for the the first time. Got the neutral ending, while also accidentally killing Rilanne during the sins incident. Planning to go to do more run throughs. It's been really fun.
Also, I saved doing the last Stairway to Heaven till near the end, and found I can't use any of Celeste's skills or magic. I can't even use her healing skills outside of battle. I noted from the corresponding page number (and the fact Rillane's sprite is shown though she doesn't speak because it's after she was killed by Malayna) that this was meant to be done quite earlier from where I was at that point, so I'm wondering if this the game getting confused because I waited too long.