Gravis, I made a blog for your question.

Your question:

I do ponder the rationale for heating only part of the filament.  However, it might be just as easy to use a tiny linear actuator to adjust the laser position than use a rotating lens to change the beam path.

My answer:
If you apply too much heat to the previous layer, the structure could collapse.
Even more heat and it could burn or a reaction will take place.
I guess to keep the structure intact, Stratasys does not expose the edges.
For first experiments I would not even use a linear motor but just make straight tracks and pre-expose them with a fixed laser. A laser with a tiny linear actuator is indeed much easier to make.

A laser prism scanner is able to expose the previous layer with a much smaller bundle.
I expect they now use a bundle diameter of 180 micrometers for a nozzle of 300 microns. (0.6*300 microns).

I think a laser prism scanner has two advantages;
  - if you swap nozzle you don't have to change focus of the LASER.
    For a 1 mm wide nozzle your bundle width would have to be 0.6*1 = 600 microns.
    A laser scanner would just project a longer line. Stratasys could solve this by
    adding a motor to the laser lens.

  - a laser prism scanner allows you to apply heat much more precisely due to the smaller laser spot.  
    Using a laser prism scanner you should be able to go well below 60 microns in bundle diameter.
    The exposure is more uniform. In the case of a single spot it is a Gaussian. With a laser scanner
    you are able to overlay multiple Gaussians, i.e. spots, and achieve a higher uniformity.
    I could imagine you need to ensure only the top of the previous layer is heated. If your molecule
    chains have a length smaller than a micron, I think 10 microns deep should be sufficient.
    It will depend on how heat is absorbed by the structure, the bundle diameter of the laser and
    diverging hereafter.  The thermal diffusion coefficient and cooling of the structure with respect
    to the air / environment.
    When the new layer is added you want the large molecules to mix at the edges and not the
    structure to collapse.
    You could look at the scattered light using OCT / laser microscope to get an idea of the  
    temperature of the structure and ensure you r not overheating.