Your parents are grown adults and should be dealing with their own matters. These issues are not your concern to remedy.
It appears as though there is too much faith being placed on Elders and the Watch Tower to enforce honesty and to make matters right. The abuse confession to Elders is worthless unless they actually called the legal authorities to be involved.
If there are threatening, accusatory and defamation issues, these need to be addressed by your parents through legal authorities including police and a qualified lawyer. Elders will do and can do nothing except play favourites.
People make all sorts of threats, especially when they are cornered. If those who threaten legal action actually do consult a lawyer, they often find that there is no basis, validity or provability to their accusations.
As your parents are partners (I assume on-title) in owning the house that was built larger than it should have been, your parents will be involved if there are any penalties assessed against the house owners by the tax and/or building department.
As a building permit and inspections will have been required, the plans for the 'legal' space would have been misrepresented, while the 'undeclared' space would not have been inspected and may not be in conformance to building safety, fire safety or electrical safety requirements.
In my province, the building department have authority to require any modifications performed without a building permit, to be torn-out and the structure restored to how it was prior to modification. Of course, restoring to the pre-existing condition usually also requires a building permit.
There may be grounds for a claim against the county building inspection department as 1000 sq/ft of additional undeclared space, should have easily been recognized by the inspector. He/she should not have been fooled by a fake wall hiding the space, as was stated in your other thread.
Your parents need to discuss the house situation with a qualified lawyer to obtain advice on how to proceed and also reduce their liability. Delay in dealing with the house issues, may be compounding their liability.
Is your mother a qualified architect, designer or technologist? Did she design the house to include the additional undeclared space or was that added by your Aunt & Uncle without your mother's knowledge? Was there a prior fee for service that was agreed and documented prior to your mother commencing the drawing preparation? A verbal agreement is an agreement but is much harder to prove compared to written documentation or contract.