Randi,
For your first point have you looked at using Observation areas instead of observation lines? The observation areas have the function CalculatePowerSpectrumTotal which I believe would fit your requirements.
I am not entirely certain what you are asking in terms of capabilities for the point source. Are you asking for a point source to independently excite multiple grid points within your unit cell? Have you looked at setting up a script that can move your source for each simulation?