I would have to agree with Fullwings on this (constant IAS = constant glide angle). I see the logic that you are trying to use, Old Smokey. I think you are trying to equate vertical descent rate with drag, thus believing that the vertical descent rate doesn't change. I don't think this is a valid assumption. There are different ways to think about this: Here is one way which may help: IAS is really the dynamic pressure. Lift is a product of dynamic pressure and angle of attack. The glide angle depends on the ratio of lift to drag. At constant IAS, the lift and the drag don't change, thus the glide angle remain constant. For what it is worth, I have run these profiles on the microsoft flight sim. The descent angle is constant with the constant IAS descent. Now this assumes that they have used valid code in the simulation, but since these codes are widely available I don't expect it's a problem.
Regards