In respect of the rebound springs, the setup ought to be that the rebound spring is not preloaded when static with the full weight of the car on the springs. Certainly on my Roadster this was not the case, although my new rebound springs were a half inch longer than the originals, presumably due to settling. If you reduce the length of the rebound spring, the main spring will expand which will increase the ride height at the front. So to retain original ride height you would need a shorter main spring. I’m not an advocate of cutting springs, because the ends are profiled to spread the load evenly and if you cut some off, it will localise the loading. A better way would be to space the bottom plate with an aluminium spacer, however the thread length at the bottom of the kingpin is limited, so it would make sense to split the kingpin spacer, half at the top of the pin and half at the bottom (if you can picture that).
Where the rebound spring is compressed statically, it will initially assist the main spring under bump until it is fully extended. The servo effect won’t be linear, it will reduce to zero in around half an inch. This means that the spring rate will rise in the first half inch or so, and become stiffer, and the initial softness may cause the front end wobble some cars exhibit, and reduced roll stiffness for the first half inch of travel on the outside front wheel.
On the road I don’t believe that the effect is detrimental, especially with decent dampers to eliminate the front end oscillation.
Conventional wishbone suspension doesn’t have rebound springs, but relies on rubber bump stops to control rebound, and in theory, the rebound spring could be replaced by a rubber hose, but it would probably not last anywhere near as long. The thickness of the rubber is limited by what will fit inside the cross tube hole.
In the case of the gap between stub axle and rebound spring, that would be just about “OK” if both sides were alike, but on a road with lateral corrugations, odd gaps will allow one wheel to drop faster than the other which is not a stable set up. It implies a faulty or broken main spring.
