[Mathew]

It seems to me that many Golf Stroke Engineers stress that the swing only goes up, back, and....then down, out, and forward. Three dimensional. Also, forward is a result of two divergent forces (down and out).

Yup that is because the golf stroke takes place on a tilted plane.

1)Regarding the arms: If an arm is straight, is this as far 'out' as the arm can go during the swing ?

The left arm is extended by extensor action but this is seperate from the geometry of the clubhead orbit. Because impact is before the lowest point (approx vertical to the ground in relation of the left shoulder) the clubhead is going downwards and just as importantly - outwards.

2)From the top of the backswing, where is the aiming point in relation to being down, out and forward ?

The aiming point is always a point on the plane line so that the thrust is onplane. The plane line always remains the same regardless of the plane angle...