OK, after some experimentation, I have found the exact order of actions required in the alchemy lab:
show paper to professor, s, z, z,
break line with knife, exit pentagram, move bench,
open trapdoor, enter trapdoor, turn on light. The problem is that you have limited time to do these actions, and the punishment for doing anything else is instant death. Much reloading was required. Getting back to the lab, I put the hand into the vat for a while to reanimate it and showed it to the urchin to scare him and get his bolt cutter. This came from a FAQ, I'm pretty sure I would never have figured that out myself. What I did figure out myself was that you can pry open the elevator doors, and in the basement, you can get below the elevator. There are some bricks loose in the wall, which I guess could lead back to the steam tunnels. Then I could probably get more items down there. Maybe use the bolt cutter to cut the tentacle things that are connected to the poor urchins?