yes

branching is possible, but only with proper planning. Each "branch" has to not only have its own chapter, but it has to have it;s own audio track as well as recut movies don;t just re-insert clips but re-edit the audio for smooth transitions or music playing smoothly through an extended scene. This is very complicated, and expensive from a manpower and creation perspective, not to mention manu DVD players don;t play nicely with this (you know the "pase" when disks switch between layers in a long movie? Those pauses are usually at natural cuts, but imagine them mid-scene, irritating no?) DVD players were not built with a proper read-ahead buffer and handle moving the disk heads mid-scene very poorly. BR is completely capable of this, but studios don;t do it as the labor and planning costs often completely outweigh the material costs in adding the additional disk (especially when a 2 disk almost always sells for a higher price than 1 anyway because most consumers would not understand a 1 disk/2 movie pricing model).

Easily possible but not as marketable?

It would be very easy to do that, quite a few discs allow for a prior selection of film running order. Memento allowed you to view all the 'chapters' in reverse order for instance.

However, would it be as likely to sell as a 3 disc edition which only costs a little more to produce?

The perceived extra value is probably the reason for packaging it this way.