Well, it's no easy task, asking for Kudos, here in IdeaStation. But let's try it .. 
For introduction, look here:
http://forums.autodesk.com/t5/inventor-forum/having-problems-turning-complex-solid-into-shell/td-p/6...
I tried this 5 mm shell both in Inventor 2016 and Fusion 360, but no luck. Shelling or surface offsets failed in general.
But an attempt with 2 mm offset in Rhino was successful, and could be imported back to Inventor.
During a closer look I've seen something interesting:
Rhino, too, seems to be not able with offsetting all faces. But Rhino shows, what can be done. See pictures:

The above yellow faces are the outside shape. Now the next one:

The green faces are offset with 2 mm to the inside. Most faces could be done, but a total of two faces were missing. In the picture, one of them allows a look to the 5 mm offset. The other one is at the backside. Another picture now with 5mm offset:
Now can be seen, that some more faces refused offsetting, but the result still is pretty good. With some boundary patches for the missing faces the shape can be closed.
And these are ways, that I'd like to see in Inventor, too.
Thanks a lot for Kudos, if you like it.
Walter