The fact they are being difficult, and refusing the cash to give place back would have me concern and go on defense to do everything by the book on this one.

Keep plenty of cash (or highly liquid, safe securities) reserves - I keep a minimum of $50k for my 10 units.Your cash flow is first line of defense.

In the plumber's defense, it could well be the foundation for this addition was not done properly. 

But there could always be specific circumstances where a shorter (or longer) hold period are appropriate.In your case an unsolicited offer to buy at an advantageous price would certainly provide a great defense if ever questioned. 

In all cases the comps the appraisers used obviously demonstrated the appraiser's lack of knowledge of the area, so I suspect the comment of @Josane Cumandala is correct - they're using unqualified saps who got a mail-order certificate from somewhere.In any case, there's no real defense against this.

I kind of feel like realtors get defensive about Zillow, which is ignoring the reality of a changing world.

Are you on offense or defense?

The beauty of the liability policy is that is will also pay all defense costs, which can get very costly, so best to put that expense on the insurance company. 

This section states that requires the Owner to pay the defense fees and any settlement or judgment (regardless of whether the Owner's insurance covers it) except in the case of willful misconduct or gross negligence.

Now you've got the SEC on your back...You need to get your business structure, paperwork and processes looked at carefully by a good real estate and business attorney... and get ahead of this.. before you're paying big fee's on defense attorneys.However, be careful when you market your deal to an end buyer or investor.