When the suspension botoms you have no suspension. That’s not good. In the VW case, in front, that will tend to make it plow.
A bottomed-out shock is not acting like a sway bar. Sway bar limits body roll.
Not following you on the “preload applied to the bottom tube.” You’re not trying to run a beam with the center grub screws other than parallel, are you? ‘Cause that might not work, or it might…but it’s going to be mighty hard to figure out along the way.