Mk, so how I would do it is make a blueprint of the splines and have a nextNode paramater of the same blueprint. The box would have to have the first spline and move to it, once done it set’s it’s movement to the next spline and starts to move towards it. You can get an example of something similar from the Unreal youtube channel here: Basics of AI | Live Training | Unreal Engine - YouTube
The random spawning could be when the box reaches the next node it spawns another ±50ft forward or backward and ±50ft up or down.