The second aggregate example looks like this:

define RolledOutIntervals:
  MedicationRequestIntervals M
    aggregate R starting (null as List<Interval<DateTime>>): R union ({
      M X
        let S: Max({ end of Last(R) + 1 day, start of X }),
          E: S + duration in days of X
        return Interval[S, E]
    }) 

If I understand it correctly, however, it will always result in null – because you specify the starting value of R as null – and null union anything is always null.  So the null will just propagate all the way through the aggregation.  You probably want R starting (List{} as List<Interval<DateTime>>) (or something like that).