Renate,
Congratulations on a nicely written essay. I too agree that quaternions, and the algebra they represent, SU(2), have the potential to reveal certain physical relationships are are essential. It's no accident that very much work in elementary particles is done according to that.
Higher dimensional forms for relativity, such as Kaluza-Klein theory are an interesting alternative to look into. I believe the "capu nili" difficulty you mention is related to the splitting of the Lorentz transformation into parity classes depending on whether the transformation is proper and orthochronous. That is dealt with in the essay Is Kinematics Compatible With Field Symmetries?
Now the especially interesting thing you can see in that essay is that the higher dimension of mass normally associated with Kaluza-Klein type approaches collapses the common space-time relativistic relations back into equations of 3 dimensional space showing the dispersion of energy while at the same time being equivalent to the 4 dimensional expressions!
Best regards,
Steve Sycamore