Surely surfaces and splines are two separate things. If you create a forest scatter on a surface (polygons) and go to custom edit mode, you will get a warning
Switching to Custom Edit mode will disable Area/Distribution parameters. Are you sure?
But the part I find confusing is you say you cannot find the option to distribute along the spline any more... but you used a surface, not a spline???
If you have spline on a surface and the surface has a forest pro scatter, in custom edit mode. Just add another FP scatter for the spline. You can add many scatters to a single surface or spline and some can use custom edit and others not