6k..wow..good luck…but you could use track camera , and then have a duplicate of her being silhouetted on top…
Shadows are tricky with green screen footage…even hollywood movies have a tough time with this..but a tinted copy of vc reflect can do a decent job if you play with the settings…you could also turn the layer 3d add a light that casts a shadow on ground plane.
Bonus question,…You should key the footage, then pre render out a png sequecne with alhpa and import that back in and then do your effects on that.