Everything makes perfect sense except (maybe):
...Span does not effect angle of attack, the rudder effects the angle of attack...
When the boat points to a heading but the actual course is to a slightly different one (usually downwind), the foils are not working at zero angle of attack (or their intended incidence angle).
When the submersed span isn't enough to provide enough lift to match the lateral component of the sail's lift, the boat will drift to leeward at a speed that will increase the foil's angle of attack until they (with the hull, as with conventional boards) provide enough lift to match the lateral component of the sail's lift.
When the submersed span is excesive and provides more lift than the necessary to match the lateral component of the sail's lift, the boat will drift to windward at a speed just enough to decrease the foil's angle of attack until they (with the hull - very bad) generate just the necessary lift to match the lateral component of the sail's lift.
If this makes sense, the submersed span directly affects the foil's angle of attack. The angle of attack is arctan (driftspeed / boat speed).
