Ok so you’ll use a pawn.
You’ll lower everything else to be below the scene node. Parent the camera directly to the scene node, and do that with the pawn as well- don’t parent the camera to the pawn.
All of your inputs, have them affect the highest point in your pawn (probably your collision) and it will move the rest of it. CLAMP that movement to whatever distance you want in X, Y(probably 0), and Z, and boom! (get this done before the spline part).
As far as movement along a spline, there are multitudes of videos for that, but to type it out would be a long process considering I know you know quite a bit of the steps involved already.