and that yaw is the angle between the longitudinal axis and the line of travel
Not the "line of travel", no. I'm not sure why that would ever be different from the direction of the "relative wind", unless you're trying to distinguish between velocity over the ground and through the air.
The problem with using the "line of travel" is that it changes if the aircraft is going in anything other than a straight line. In physics terms, it's not an
inertial frame.
Does it all matter from the point of view of explaining EoC to a student? Probably not, but then as I said at the start of the thread, I think the best and simplest explanation is roll => bank => slip => yaw to explain why you end up with a heading change after some aileron input.