Leave the extension in the model. I can explain how to split bodies to get a result with and without the extension, but I need to know if you are using DesignModeler or SpaceClaim to edit your geometry, since the steps are different.
What output are you tracking when you try different boundary conditions?
How much difference is there between the different boundary conditions?
Please reply with images of the model and the actual values of the result.
Recall the discussion 4 weeks ago where I showed there was a 35% difference in deformation between a shell model and solid model? I didn't have to worry about that because I chose one model and I adjusted the Shear Modulus until that model matched the experimental data. So just choose a boundary condition and tune the model until it matches the experimental data.
It's great to learn how to use contact, but you don't need it for this model.