If I understand the rest or you say tool course then the only way I know (my little forum mates who are strong ;-) will give you others).
The best is to redo a spline in one piece (for the trajectory it's preferable) by guiding you on the different points of the different splines. This normally puts an automatic or manual constraint. Then you can turn the new spline into a trajectory
To make it easier: first convert the "model splines" into a construction line (with right mouse click).
On the other hand, can the various existing splines really be used to make a tool trail is also the question.
Hoping that my explanation and my questions don't make you spleen ;-)