The way you have your experiment linked, you will not be able to randomize that event alone because it is attached to the same trial as the other events.
Regarding your question about different participants getting different stimuli, you can use the Participant Groups feature to do this. Participant Groups is found under the Experiment menu. When you go to run the experiment you would choose the participant group you want when the Run Experiment window comes up.
Your question regarding marking incorrect responses, yes this is possible within SuperLab. Please see the SuperLab Manual under Chapter 3. There is a section called “Determining the Correct Response”. Under this section it explains how to setup correct and incorrect responses.