is this all one part that you're trying to "Extract" as a single midsurface?
this will unlikely be successful, most likely you should:
1. make this simpler in general
2. create your own surfaces to define the mid surfaces that are important
Mid surface extraction was easy in SDRC I-DEAS (many moons ago) but I believe it is way beyond SW capabilities. What if you re-modeled that part using just surfaces (the mid ones) and apply the FEA using shell elements? As an alternative you might try simplifying some of the geometry in non-critical areas and add shells in specific areas of interest. Some of that in the lower right of your pic looks as if it might be cosmetic and could be represented by a uniform wall, IDK. Also, if you take out as many radii as practical it will simplify the meshing just watch for stress risers.