I used the smoke powder pouches I found in the hidden room. They (stupidly) leave a burning surface (what does burn, the stone?), it may be really as simple as that that Nere becomes hostile if he touches the fire. Maybe I was just lucky the second trial. As the game hints at using smoke powder to free Nere, Larian should take care that it's use cannot foul the quest. I will use bombs in the future (which are seemingly filled with inflammable smoke powder).
