So, out of curiosity, I tried playing through the whole quest from scratch, starting from before the point I approached the noble lad at the tavern but after I had the encounter with the crimp in the church. I did all of the same things, the only differences that I can recall being 1) I did everything a lot more quickly as I already knew what to do and 2) I learned about Lucas’s secret from the Abbot’s book only before confronting him about it. Lo and behold, this time I could accuse Pious directly, no poisoning or murder. Just to confirm, I reloaded my old save to see if I was just missing the option, and it really wasn’t there. I am still frustrated, but it does turn out to have been a bug. I still feel that the quest needs some work, but this addresses my biggest complaint.
frelmedieval: I agree. It would have been nice to have some way to disguise yourself, and the monks and bandits are both too credulous to be believable (even monks aren’t that ready to believe!).