You get to specify that the trolley doesn’t have brakes because you can say “I will only apply the conclusion I am getting from the trolley problem in real life situations that are similar to the trolley problem in that there are no (metaphorical) brakes.”
The equivalent for your slavery contract is “I will permit slavery only in situations where I will starve if a slavery contract is not allowed”. You can’t do this, because you can’t have a slavery law that applies only in a specific, unlikely, scenario. A law permitting slavery will apply in the vastly more common scenario where the slaver is still incentivised to get some money from you and sell you the food.
That allows the trolls to control the direction of the discussion by picking a topic and having other people add new information to it. Some topics are inherently inflammatory or inherently are likely to attract people who behave badly, so even letting the troll pick the topic can be a disaster. You do not want to have a troll arguing that Jews descend from Cthulu, provide “new information”, and attract people sincerely arguing that Jews are not descendants of Cthulhu but are as evil as if they were, or even just Holocaust deniers.