set_process(true) hasn't been required since Godot 2. You may be looking at very old documentation/tutorials.
offset is the distance that the Pathfollow2D is located along the path. Your code, more succinctly written as:
follow.offset += 350 * delta
Is increasing that value by that amount every frame. To go backwards, you would just subtract instead of adding.
If you start with
offset = 0, you'll be at the start of the path. To start at the end, you need to get the length of the curve:
follow.offset = curve.get_baked_length()