as for the heave spring
what you say it´s correct to a point, the systems you discribe is common when a heave spring is combined with a corner spring.
In this case the two springs work in parallel,
where there can be a zone of "no engagement" for the heave spring ( like a bumpstop gap).
But technically it´s no problem, to use a two stage heave spring, or any combination. A heave spring does not need to be mega stiff, and can change it´s stiffness during compression/travel.
As DaveW pointed out, one advantge can be, that the springing of the two modes (roll/heave) can be tuned indepentend of each other.
A "corner spring" will work in roll&heave so it´s affecting both at the same time.
If you only wanted to stiffen the car in heave, and do it with the corner springs, you would need to soften the ARB by the same amount, if you don´t want to affect the roll stiffness at the same time.
As allways, there are different ways to skin a cat, and different people (teams/designers) will go down different routes to achieve the best compromise they see fit.
"Make the suspension adjustable and they will adjust it wrong ......
look what they can do to a carburetor in just a few moments of stupidity with a screwdriver." - Colin Chapman
“Simplicity is the ultimate sophistication.” - Leonardo da Vinci