i Cant join the faces without messing things up.
PLEASE HELP.
Are you trying to join the wall with the center piece? If so have you tried to loft the 2 faces?
Try what @SCALEMODEL suggested, or, try using Replace Face.
I think it’s not going to work. It looks like the top faces were tilted outward with the move tool. Jannas second image is the clue for that reasoning. Look at the warping near the corner. I believe it will need to be redrawn a bit differently. I could be wrong though.
try to use the loft tool, from one surface to the other one 
//L