The secret here is to use delete face with heal on and delete the inside of the punch first and only afterwards the outside faces of the punch in a separate delete operation. You can select inside faces of all punches for your first delete face operation.
If your vendor exported an assembly with one part, Inventor will imported it as multi-body part with one body and it will complain that it can't be converted to sheet metal.
In this case you can export it again to a neutral file format and import that back or you can use derive to get it all as one body.
I have a test model for you made with Inventor 2014 where you can try these out.
When you try and convert to sheet metal you will see that inventor complains about having multiple bodies so we need to derive it into a single body.
1. Open a new part.
2. Go to 3d model / create/ derive and browse to our file : 141017 Sheet metal imports.ipt
3. In the derived part dialog, expand solid bodies, and unmark Body.1 (you only need to have Body.13)
4. On the derived style select single solid body and click ok.
Even though we have only one body it seems that the round inserts at the end of the arms are modeled in so we need to remove them to a constant thickness part.
5. Extrude-cut the sheet metal nuts (round inserts)
6. Select the inside faces of the punches, one at a time or all at once. Start the delete face operation and after marking heal option click ok. You should have something as in the images bellow.
8. Verify the thickness and change it accordingly in the sheet metal options.
9. Use flat pattern. You might need to select one face (inside face) before flat patterning if it doesn't unfold. You might also need to delete the existing flat pattern if any to get it to compute.
And of course a video.