I was pulling my hair out until I found this. The Shadowcasting algorithm is exactly what I wanted. You're a hero! If I ever release my Rogue-ish project, I will credit you. Meanwhile, THANKS!
You can't save during missions, no. It's a divisive design choice, but a deliberate one. I've tried my very best to account for it when designing the systems and levels. Missions are pretty short, especially the early ones, and getting spotted is far from a game over ;)