I opened the part and found a lot of disjoint bodies.
isn't it much simpler to just draw a sheetmetal box and make
a punch for the B.
The B is also not optimal, sharp unclean edges.
You can sew the faces together but when selecting the tool do it manually.
The right click in your part navigator an click timestamp order. Then you can see
where these disjoint bodies are. They look very small
the autocad file doesn't have decent geometric constraints so the shape will not be a nice one.
The is the left corner very sharp, when you use deep drawing it can become a problem (also the
inner corners in the B
You need something like this
reuse with modeling tools the contours of the B that you find the most interesting.
If you want to make it as a cutout you might want to use "project curve" in a new sketch. Then use that sketch to either make a normal cutout or a new extrude.
You could also reuse the face of the "B" IGES and perform a thicken+ unite. The deformation itself will not flatten, but at least your basic box will be flattened. and the flat pattern wil contain the geometry of the "B"
Can your IGES supplier also give you a parasolid or STEP file? That would at least give you a solid body to work on.