Okay i think i found it, after checking back with everything and returning to the Combine Rotator function i ensured that the Rotation for the turning left and turning right is the “basis” for the calculation → the first rotation in “Combine Rotator”
And it actually worked… no rotation of the spline itself or any kind of it screws the rotation of it, even with me using all the additional values (following picture)
btw the final calculation looks like this:
Thats for helping me on that one 
The only thing that comes to mind that led to this, was me using the wrong order for combine rotator, which considering how much time i spend on this and the goose chase that ensued, is realy way to much