I think this is where things get murky...you say inserting a spacer makes things stiffer. That might be the case for a progressive spring, but what I was describing above was for a constant rate spring. According to what I copied, a spacer just adds preload and it affects sag...does not make for a stiffer spring. If you have an effective 200 poungs of preload, you must load it higher than 200 to make it begin to deflect again.
In these situations, isn't the spring (with spacer) pushing back against any further force and that you must overcome this pushback before the spring moves anymore? Using Hooke's law for a constant rate spring, with a spring rate of 100 lb/in, the 200 pounds (due to the spacer) will deform the spring 2 inches. If we put 200 pounds onto the spring/fork setup, the restorative force will just balance the 200 pounds of added weight and the spring will not move. If we increase the 200 to 300 pounds, the spring will deflect another 1 inch for 3 inches overall. But this still satisfies the original equation for a spring rate of 100 lb/in...300 pounds deflected the spring 3 inches.
Progressive springs get more complicated, but the situation is still the same...the preload creates the restorative force...you must over come that with more force created during preload in order to deflect the spring any more. That's why you increase spring preload, say with the knobs to turn on the outside, when riding with a passenger or when traveling with all your gear.