How plausible (and how hard) is it to modify a billy bookcase to fit a gable-end corner, floor-to-ceiling, with an impinging 45deg roof
None of this would be considered trivial.  For example, just to do the left bookcase conversion for the 45-degree top.
After cutting off the top of the cabinet to make the 45-degree angle, you would not have a top piece to place on top.  The top board is now too narrow because it is at a 45-degree angle.  The first issue is how to replace or extend the depth of the top to fit the top of the case.  The new cut on the sides no longer have the holes for the cam screw.  You would have to replace the original cam screw with other fasteners to join the new top to the sides. 
The corner unit adds further complications.
If possible, I would look to find a wall unit that fits up to the gable.
I just modified a full-size pantry cabinet to fit under a stairwell with the angle going across the full width of the cabinet, and it is not trivial.