Reviewing through the discussion you linked I think you were on the right track in answering your questions. Please find my notes below:
Q1: Could you please let me know how to change the default of the checkbox, or direct me to some relevant content?
HIgh level customization info from the developer:
So, while this is not an option in the out of the box product, you should be able to accomplish this type of modification through a customization to your instance. If you would like assistance developing customizations for your instance I would recommend submitting a case in your customer group here in the Jive Community or reaching out to your Account Manager who can assist in setting up an engagement with our Professional Service's team. They will then be able to work with you to determine the scope of the project and develop a custom feature which meets your use case.
Q2: Is it possible to change this default for individual documents?
This could also be a part of the custom option, however is not one that is available in the default platform.
I had a feeling that it would require some more advanced coding/ professional help. Your reply has helped me a lot, and this is definitely something worth looking into with my peers.