Vibration after lowering on the rear - too low?

Please elaborate.
Swaybars act in roll, not in pitch, don't they? Why would they have preload if both wheels go up the same amount?

Please elaborate.
The spring rate doesn't change when you compress or release the spring (unless the coils touch), that's why it's called a rate - unless you have progressive/digressive springs or geometry. The preload can be changed but not the rate - when you increase the preload, your car sits higher and vice versa. The rate doesn't change.

It's not as the rate hasn't changed, only the ride height via the preload or different spring mounting height. The dampers don't know nor care about the preload or mounting height. Generally, the dampers display the same characteristics regardless of piston position except when you get to the very ends of the stroke. Even with a lowered car the damper will still work the same way.

Please elaborate.
Are suggesting that by changing the rake within a few mm you can significantly change the weight distribution?? It won't. Do the maths.
Hint: On my Mondial, lifting the front 93mm (ninety three!) moved the CG o.3% (zero point three) to the back. That sort of shift of CG won't cure any vibration issues on a road car. And it looks stupid, not what the people here try to achieve. I lifted the front end to calculate the CG height by the way.