Hasta La Motion!

I’m a french student in a VFX school and I’m in a dead end here.
I’m working on a short movie which is set in the Middle Ages.
I would like to warp shadows from a shot to make it match to a matte painting I made (WIP).

Here is a part of the shot to illustrate what I’m trying to do:

Original shot:

Matte painting:

Shot composited (here quickly in photoshop for the example):

The problem is that I want to keep the beginning of the shadow on the floor (less rotoscoping) and then warp it on the walls. I have a 3D layout which can be used for porjection.

Any idea?

Thanks in advance