I got your example to work by specifying an array for slope instead of using the default of an empty array. I would appear that there's something in the call library node function that doesn't process channel 0 correctly. I would call it a bug since channel 1 seems to work. As far as getting pretrigger data, since you are getting the time of the trigger and then using that time for the Get Waveform Subset, you should be able to subtract a given time amount from the trigger event time and use that as the start time.
