I managed to get Nersmias's conviction at -1 but the conversation continued and when I ended it, I still had to make a "deal" with him for me to talk to Sillan :(
I reviewed the event and didn't see anything strange. However, if Nersmias' trust reaches 0 or less on the same round his conviction reaches 0, you will be locked out of the route to talk to Sillan at no cost.