Back again! For what it's worth, I was able to get back to playing by unplugging my external monitor. When I started PoE up on just the laptop, it was running exactly as intended, fullscreen at 2560x1440 resolution. From there, I just put it into windowed mode again via the in-game controls and plugged my external monitor back in. I still can't get it to be fullscreen, but now at least I can play the game again in windowed mode!